Make a service model and use for permissions

Having the service floating about as JSON is a bit flakey. Could easily
introduce a mistake where you mistype the name of a key and silently
get `None`.

Also means doing awkward things like `if 'permission' in
current_service['permissions']`, whereas for users we can do the
much cleaner `user.has_permission()`.

So this commit:
- introduces a model
- adds a `.has_permission` method similar to the one we have for users

app/main/views/api_keys.py

@@ -37,7 +37,7 @@ dummy_bearer_token = 'bearer_token_set'
 @user_has_permissions('manage_api_keys')
 def api_integration(service_id):
     callbacks_link = (
-        '.api_callbacks' if 'inbound_sms' in current_service['permissions']
+        '.api_callbacks' if current_service.has_permission('inbound_sms')
         else '.delivery_status_callback'
     )
     return render_template(
@@ -104,7 +104,7 @@ def create_api_key(service_id):
             'Not available because your service is in '
             '<a href="{}#trial-mode">trial mode</a>'.format(url_for(".using_notify"))
         )
-    if 'letter' in current_service['permissions']:
+    if current_service.has_permission('letter'):
         option_hints[KEY_TYPE_TEAM] = 'Can’t be used to send letters'
     if form.validate_on_submit():
         if form.key_type.data in disabled_options:
@@ -198,7 +198,7 @@ def get_delivery_status_callback_details():
 def delivery_status_callback(service_id):
     delivery_status_callback = get_delivery_status_callback_details()
     back_link = (
-        '.api_callbacks' if 'inbound_sms' in current_service['permissions']
+        '.api_callbacks' if current_service.has_permission('inbound_sms')
         else '.api_integration'
     )