migrations: Add migration to fix Attachment cache.
migrations: Add migration to fix Attachment cache.

This migration needs to be run after the previous commit is deployed
to a given Zulip installation, to fix any stale values of
is_realm_public and is_web_public.

rht/zulip,rht/zulip,rht/zulip,rht/zulip,kou/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,zulip/zulip,kou/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,kou/zulip,kou/zulip,andersk/zulip,andersk/zulip,andersk/zulip,andersk/zulip,zulip/zulip,kou/zulip,kou/zulip,rht/zulip,kou/zulip,zulip/zulip