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/user_agent.py

@@ -1,9 +1,11 @@
 import re
+from typing import Optional, Dict
 
 # Warning: If you change this parsing, please test using
 #   tools/test_user_agent_parsing.py
 # And extend tools/user_agents_unique with any new test cases
 def parse_user_agent(user_agent):
+    # type: (str) -> Optional[Dict[str, str]]
     match = re.match("^(?P<name>[^/ ]*[^0-9/(]*)(/(?P<version>[^/ ]*))?([ /].*)?$", user_agent)
     if match is None:
         return None