alert_words: Move alert_words from UserProfile to separate model.

Previously, alert words were a JSON list of strings stored in a
TextField on user_profile.  That hacky model reflected the fact that
they were an early prototype feature.

This commit migrates from that to a separate table, 'AlertWord'.  The
new AlertWord has user_profile, word, id and realm(denormalization so
we can provide a nice index for fetching all the alert words in a
realm).

This transition requires moving the logic for flushing the Alert Words
caches to their own independent feature.

Note that this commit should not be cherry-picked without the
following commit, which fixes case-sensitivity issues with Alert Words.

zerver/lib/message.py

@@ -58,7 +58,7 @@ from zerver.models import (
 from typing import Any, Dict, List, Optional, Set, Tuple, Sequence
 from typing_extensions import TypedDict
 
-RealmAlertWords = Dict[int, List[str]]
+RealmAlertWord = Dict[int, List[str]]
 
 RawUnreadMessagesResult = TypedDict('RawUnreadMessagesResult', {
     'pm_dict': Dict[int, Any],