Add type annotation to mention.py, redis_utils.py, timestamp.py, user_agent.py

Some functions in models.py had input typed as int when they needed to be typed as datetime.datetime

zerver/lib/mention.py

@@ -7,4 +7,5 @@ find_mentions = r'(?<![^\s\'\"\(,:<])@(?:\*\*([^\*]+)\*\*|(\w+))'
 wildcards = ['all', 'everyone']
 
 def user_mention_matches_wildcard(mention):
+    # type: (str) -> bool
     return mention in wildcards