Add button to approve broadcast

Since new broadcasts will go into `pending-approval`, we now need a way
of approving them.

This commit adds a button to this page to start (or approve) the
broadcast. This button is wrapped in a bordered box, to emphasise that
it’s something consequential.

app/models/broadcast_message.py

@@ -138,6 +138,12 @@ class BroadcastMessage(JSONModel):
         )
         self._set_status_to('pending-approval')
 
+    def approve_broadcast(self):
+        self._update(
+            starts_at=datetime.utcnow().isoformat(),
+        )
+        self._set_status_to('broadcasting')
+
     def cancel_broadcast(self):
         self._set_status_to('cancelled')