Remove, cache or hardcode anything that could be cached when sending an email

Probably shouldn’t deploy this to production 😅

This shows exactly what I removed, bodged and hardcoded to test what the
performance implications of caching a bunch of stuff might look like.

Test command:
```bash
python -m timeit -n 1 -s "import notifications_python_client; c = notifications_python_client.notifications.NotificationsAPIClient('🤫', base_url='http://localhost:6011')" "c.send_email_notification(email_address='sender@something.com', template_id='be433bfc-fe31-464b-9f2c-5be11abf2176')"
```

Before:
```
raw times: 12.1 11.9 11.8
100 loops, best of 3: 118 msec per loop
```

After
```
raw times: 11.2 10.7 10.1
100 loops, best of 3: 101 msec per loop
```

Not a big improvement… So I was curious what it was doing in those
~100ms

Let’s go back to master and comment out persisting the notification to
the database:
```
raw times: 12.3 10.5 10.5
100 loops, best of 3: 105 msec per loop
```

(saves about 13ms)

If we comment out sending to the queue:
```
raw times: 3.43 3.24 4.88
100 loops, best of 3: 32.4 msec per loop
```

(saves about 85ms)

This means most of our request time is spent waiting for SQS.

If we test our fake caching while sending to the queue is commented
out we get a clearer picture of the potential improvements:
```
raw times: 2.13 1.84 2.18
100 loops, best of 3: 18.4 msec per loop
```

This is a saving of 14ms, from a baseline of 32.4ms, so 56%.

A typical call to fetch a service from Redis from the admin app takes
about 0.6ms, for context.

It’s also worth thinking about whether we’re holding a database
connection longer than we need to if we still have it while talking to
SQS.

app/v2/notifications/post_notifications.py

@@ -121,20 +121,12 @@ def post_notification(notification_type):
     if notification_type == EMAIL_TYPE:
         form = validate(request_json, post_email_request)
     elif notification_type == SMS_TYPE:
-        form = validate(request_json, post_sms_request)
+        abort(400)
     elif notification_type == LETTER_TYPE:
-        form = validate(request_json, post_letter_request)
+        abort(400)
     else:
         abort(404)
 
-    check_service_has_permission(notification_type, authenticated_service.permissions)
-
-    scheduled_for = form.get("scheduled_for", None)
-
-    check_service_can_schedule_notification(authenticated_service.permissions, scheduled_for)
-
-    check_rate_limiting(authenticated_service, api_user)
-
     template, template_with_content = validate_template(
         form['template_id'],
         form.get('personalisation', {}),
@@ -142,15 +134,10 @@ def post_notification(notification_type):
         notification_type,
     )
 
-    reply_to = get_reply_to_text(notification_type, form, template)
+    reply_to = 'test@example.com'
 
     if notification_type == LETTER_TYPE:
-        notification = process_letter_notification(
-            letter_data=form,
-            api_key=api_user,
-            template=template,
-            reply_to_text=reply_to
-        )
+        abort(400)
     else:
         notification = process_sms_or_email_notification(
             form=form,
@@ -164,11 +151,7 @@ def post_notification(notification_type):
         template_with_content.values = notification.personalisation
 
     if notification_type == SMS_TYPE:
-        create_resp_partial = functools.partial(
-            create_post_sms_response_from_notification,
-            from_number=reply_to,
-            content=str(template_with_content),
-        )
+        abort(400)
     elif notification_type == EMAIL_TYPE:
         create_resp_partial = functools.partial(
             create_post_email_response_from_notification,
@@ -177,16 +160,12 @@ def post_notification(notification_type):
             content=WithSubjectTemplate.__str__(template_with_content),
         )
     elif notification_type == LETTER_TYPE:
-        create_resp_partial = functools.partial(
-            create_post_letter_response_from_notification,
-            subject=template_with_content.subject,
-            content=WithSubjectTemplate.__str__(template_with_content),
-        )
+        abort(400)
 
     resp = create_resp_partial(
         notification=notification,
         url_root=request.url_root,
-        scheduled_for=scheduled_for
+        scheduled_for=None
     )
     return jsonify(resp), 201
 
@@ -196,7 +175,7 @@ def process_sms_or_email_notification(*, form, notification_type, api_key, templ
     form_send_to = form['email_address'] if notification_type == EMAIL_TYPE else form['phone_number']
 
     send_to = validate_and_format_recipient(send_to=form_send_to,
-                                            key_type=api_key.key_type,
+                                            key_type='test',
                                             service=service,
                                             notification_type=notification_type)
 
@@ -244,8 +223,8 @@ def process_sms_or_email_notification(*, form, notification_type, api_key, templ
         service=service,
         personalisation=personalisation,
         notification_type=notification_type,
-        api_key_id=api_key.id,
-        key_type=api_key.key_type,
+        api_key_id=None,
+        key_type='test',
         client_reference=form.get('reference', None),
         simulated=simulated,
         reply_to_text=reply_to_text,