mypy: Add EmojiInfo type.

We now serialize still_url as None for non-animated emojis,
instead of omitting the field. The webapp does proper checks
for falsiness here.  The mobile app does not yet use the field
(to my knowledge).

We bump the API version here. More discussion here:

https://chat.zulip.org/#narrow/stream/378-api-design/topic/still_url/near/1302573

zerver/lib/event_schema.py

@@ -712,9 +712,7 @@ realm_emoji_type = DictType(
         ("source_url", str),
         ("deactivated", bool),
         ("author_id", int),
-    ],
-    optional_keys=[
-        ("still_url", str),
+        ("still_url", OptionalType(str)),
     ],
 )