Make the broadcast flow talk to the API

This commit removes the code the puts areas into the session and instead
creates and then updates a draft broadcast in the database.

This is so we can avoid session-related bugs, and potentially having a
large session when we start adding personalisation etc.

Once a broadcast is ready to go it is set to `broadcasting` straight
away with no approval. We’ll revisit this as we learn more about how
users might want to manage who can create and approve broadcasts.

The tests are a bit light in terms of checking what’s on the page, but
clicking through the pages is probably good enough for now.

app/models/broadcast_message.py

@@ -0,0 +1,111 @@
+from datetime import datetime, timedelta
+
+from notifications_utils.broadcast_areas import broadcast_area_libraries
+from notifications_utils.template import BroadcastPreviewTemplate
+from orderedset import OrderedSet
+
+from app.models import JSONModel
+from app.notify_client.broadcast_message_api_client import (
+    broadcast_message_api_client,
+)
+from app.notify_client.service_api_client import service_api_client
+
+
+class BroadcastMessage(JSONModel):
+
+    ALLOWED_PROPERTIES = {
+        'id',
+        'service_id',
+        'template_id',
+        'template_name',
+        'template_version',
+        'service_id',
+        'created_by',
+        'personalisation',
+        'starts_at',
+        'finishes_at',
+        'status',
+        'created_at',
+        'approved_at',
+        'cancelled_at',
+        'updated_at',
+        'created_by_id',
+        'approved_by_id',
+        'cancelled_by_id',
+    }
+    DEFAULT_TTL = timedelta(hours=72)
+
+    libraries = broadcast_area_libraries
+
+    @classmethod
+    def create(cls, *, service_id, template_id):
+        return cls(broadcast_message_api_client.create_broadcast_message(
+            service_id=service_id,
+            template_id=template_id,
+        ))
+
+    @classmethod
+    def from_id(cls, broadcast_message_id, *, service_id):
+        return cls(broadcast_message_api_client.get_broadcast_message(
+            service_id=service_id,
+            broadcast_message_id=broadcast_message_id,
+        ))
+
+    @property
+    def areas(self):
+        return broadcast_area_libraries.get_areas(
+            *self._dict['areas']
+        )
+
+    @property
+    def polygons(self):
+        return broadcast_area_libraries.get_polygons_for_areas_lat_long(
+            *self._dict['areas']
+        )
+
+    @property
+    def template(self):
+        response = service_api_client.get_service_template(
+            self.service_id,
+            self.template_id,
+            version=self.template_version,
+        )
+        return BroadcastPreviewTemplate(response['data'])
+
+    def add_areas(self, *new_areas):
+        broadcast_message_api_client.update_broadcast_message(
+            broadcast_message_id=self.id,
+            service_id=self.service_id,
+            data={
+                'areas': list(OrderedSet(
+                    self._dict['areas'] + list(new_areas)
+                ))
+            },
+        )
+
+    def remove_area(self, area_to_remove):
+        broadcast_message_api_client.update_broadcast_message(
+            broadcast_message_id=self.id,
+            service_id=self.service_id,
+            data={
+                'areas': [
+                    area for area in self._dict['areas']
+                    if area != area_to_remove
+                ]
+            },
+        )
+
+    def start_broadcast(self):
+        broadcast_message_api_client.update_broadcast_message(
+            broadcast_message_id=self.id,
+            service_id=self.service_id,
+            data={
+                'starts_at': datetime.utcnow().isoformat(),
+                'finishes_at': (datetime.utcnow() + self.DEFAULT_TTL).isoformat(),
+            },
+        )
+        broadcast_message_api_client.update_broadcast_message_status(
+            'broadcasting',
+            broadcast_message_id=self.id,
+            service_id=self.service_id,
+        )