Expire test and operator alerts after 4 hours

While testing alerts on these channels the MNOs sometimes need to
restart their CBCs to make sure everything is failing over properly.

If the CBC does not come back up, for whatever reason, then we are left
in a state where the alert can’t be cancelled.

To minimise the impact to the public in this scenario we should keep the
expiry time at 4 hours for alerts sent on test channels. We recently
increased it back up to 24 hours for all channels, so this in effect is
reverting that change for channels that won’t be used in a real
emergency.

app/models/broadcast_message.py

@@ -248,12 +248,15 @@ class BroadcastMessage(JSONModel):
     def request_approval(self):
         self._set_status_to('pending-approval')
 
-    def approve_broadcast(self):
+    def approve_broadcast(self, channel):
+        if channel in {'test', 'operator'}:
+            ttl = timedelta(hours=4, minutes=0)
+        else:
+            ttl = timedelta(hours=23, minutes=59)
+
         self._update(
             starts_at=datetime.utcnow().isoformat(),
-            finishes_at=(
-                datetime.utcnow() + timedelta(hours=23, minutes=59)
-            ).isoformat(),
+            finishes_at=(datetime.utcnow() + ttl).isoformat(),
         )
         self._set_status_to('broadcasting')