Add boilerplate for sending SMS via Reach

This works in conjunction with the new SMS provider stub [^1].

Local testing:

- Run the migrations to add Reach as an inactive provider.
- Activate the Reach provider locally and deactivate the others.

      update provider_details set priority = 100, active = false where notification_type = 'sms';
      update provider_details set active = true where identifier = 'reach';

- Tweak your local environment to point at the SMS stub.

      export REACH_URL="http://host.docker.internal:6300/reach"

- Start / restart Celery to pick up the config change.
- Send a SMS via the Admin app and see the stub log it.
- Reset your environment so you can send normal SMS.

      update provider_details set active = true where notification_type = 'sms';
      update provider_details set active = false where identifier = 'reach';

[^1]: https://github.com/alphagov/notifications-sms-provider-stub/pull/10

app/clients/sms/reach.py

@@ -1,3 +1,7 @@
+import json
+
+from requests import RequestException, request
+
 from app.clients.sms import SmsClient, SmsClientResponseException
 
 
@@ -13,13 +17,49 @@ def get_reach_responses(status, detailed_status_code=None):
 
 
 class ReachClientResponseException(SmsClientResponseException):
-    pass  # TODO (custom exception for errors)
+    def __init__(self, response, exception):
+        status_code = response.status_code if response is not None else 504
+        text = response.text if response is not None else "Gateway Time-out"
+
+        self.status_code = status_code
+        self.text = text
+        self.exception = exception
+
+    def __str__(self):
+        return "Code {} text {} exception {}".format(self.status_code, self.text, str(self.exception))
 
 
 class ReachClient(SmsClient):
+    def init_app(self, *args, **kwargs):
+        super().init_app(*args, **kwargs)
+        self.url = self.current_app.config.get('REACH_URL')
 
-    def get_name(self):
-        pass  # TODO
+    @property
+    def name(self):
+        return 'reach'
 
-    def send_sms(self, to, content, reference, international, multi=True, sender=None):
-        pass  # TODO
+    def try_send_sms(self, to, content, reference, international, sender):
+        data = {
+            # TODO
+        }
+
+        try:
+            response = request(
+                "POST",
+                self.url,
+                data=json.dumps(data),
+                headers={
+                    'Content-Type': 'application/json',
+                },
+                timeout=60
+            )
+
+            response.raise_for_status()
+            try:
+                json.loads(response.text)
+            except (ValueError, AttributeError) as e:
+                raise ReachClientResponseException(response=response, exception=e)
+        except RequestException as e:
+            raise ReachClientResponseException(response=e.response, exception=e)
+
+        return response