Correctly include room avatars in email notifications (#10658)

Judging by the template, this was intended ages ago, but we never
actually passed an avatar URL to the template. So let's provide one.

Closes #1546.

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

synapse/push/mailer.py

@@ -258,7 +258,7 @@ class Mailer:
         # actually sort our so-called rooms_in_order list, most recent room first
         rooms_in_order.sort(key=lambda r: -(notifs_by_room[r][-1]["received_ts"] or 0))
 
-        rooms = []
+        rooms: List[Dict[str, Any]] = []
 
         for r in rooms_in_order:
             roomvars = await self._get_room_vars(
@@ -362,6 +362,7 @@ class Mailer:
             "notifs": [],
             "invite": is_invite,
             "link": self._make_room_link(room_id),
+            "avatar_url": await self._get_room_avatar(room_state_ids),
         }
 
         if not is_invite:
@@ -393,6 +394,27 @@ class Mailer:
 
         return room_vars
 
+    async def _get_room_avatar(
+        self,
+        room_state_ids: StateMap[str],
+    ) -> Optional[str]:
+        """
+        Retrieve the avatar url for this room---if it exists.
+
+        Args:
+            room_state_ids: The event IDs of the current room state.
+
+        Returns:
+             room's avatar url if it's present and a string; otherwise None.
+        """
+        event_id = room_state_ids.get((EventTypes.RoomAvatar, ""))
+        if event_id:
+            ev = await self.store.get_event(event_id)
+            url = ev.content.get("url")
+            if isinstance(url, str):
+                return url
+        return None
+
     async def _get_notif_vars(
         self,
         notif: Dict[str, Any],