Add a page to view a single broadcast

This commit adds a page to view a single broadcast. This is important
for two reasons:
- users need an audit of what happened when, and who else was involved
  in approving or cancelling a broadcast
- we need a place to put actions (approving, cancelling) on a broadcast
  so that you can confirm details of the message and the areas before
  performing the action

app/models/broadcast_message.py

@@ -5,6 +5,7 @@ from notifications_utils.template import BroadcastPreviewTemplate
 from orderedset import OrderedSet
 
 from app.models import JSONModel, ModelList
+from app.models.user import User
 from app.notify_client.broadcast_message_api_client import (
     broadcast_message_api_client,
 )
@@ -94,6 +95,18 @@ class BroadcastMessage(JSONModel):
             return 'completed'
         return self._dict['status']
 
+    @property
+    def created_by(self):
+        return User.from_id(self.created_by_id)
+
+    @property
+    def approved_by(self):
+        return User.from_id(self.approved_by_id)
+
+    @property
+    def cancelled_by(self):
+        return User.from_id(self.cancelled_by_id)
+
     def add_areas(self, *new_areas):
         broadcast_message_api_client.update_broadcast_message(
             broadcast_message_id=self.id,