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.
Anders Kaseorg
Abhishek-Balaji