add broadcast message crud

new blueprint `/service/<id>/broadcast-message` with the following
endpoints:

* GET / - get all broadcast messages for a service
* GET /<id> - get a single broadcast message
* POST / - create a new broadcast message
* POST /<id> - update an existing broadcast message's data
* POST /<id>/status - move a broadcast message to a new status

I've kept the regular data update (eg personalisation, start and end
times) separate from the status update, just to keep separation of
concerns a bit more rigid, especially around who can update.

I've included schemas for the three POSTs, they're pretty
straightforward.

app/broadcast_message/broadcast_message_schema.py

@@ -0,0 +1,48 @@
+from app.schema_validation.definitions import uuid
+from app.models import BroadcastStatusType
+
+create_broadcast_message_schema = {
+    '$schema': 'http://json-schema.org/draft-04/schema#',
+    'description': 'POST create broadcast_message schema',
+    'type': 'object',
+    'title': 'Create broadcast_message',
+    'properties': {
+        'template_id': uuid,
+        'service_id': uuid,
+        'created_by': uuid,
+        'personalisation': {'type': 'object'},
+        'starts_at': {'type': 'string', 'format': 'date-time'},
+        'finishes_at': {'type': 'string', 'format': 'date-time'},
+        'areas': {"type": "array", "items": {"type": "string"}},
+    },
+    'required': ['template_id', 'service_id', 'created_by'],
+    'additionalProperties': False
+}
+
+update_broadcast_message_schema = {
+    '$schema': 'http://json-schema.org/draft-04/schema#',
+    'description': 'POST update broadcast_message schema',
+    'type': 'object',
+    'title': 'Update broadcast_message',
+    'properties': {
+        'personalisation': {'type': 'object'},
+        'starts_at': {'type': 'string', 'format': 'date-time'},
+        'finishes_at': {'type': 'string', 'format': 'date-time'},
+        'areas': {"type": "array", "items": {"type": "string"}},
+    },
+    'required': [],
+    'additionalProperties': False
+}
+
+update_broadcast_message_status_schema = {
+    '$schema': 'http://json-schema.org/draft-04/schema#',
+    'description': 'POST update broadcast_message status schema',
+    'type': 'object',
+    'title': 'Update broadcast_message',
+    'properties': {
+        'status': {'type': 'string', 'enum': BroadcastStatusType.STATUSES},
+        'created_by': uuid,
+    },
+    'required': ['status', 'created_by'],
+    'additionalProperties': False
+}