Use log_search to track already sent thanks
We currently use client-side storage to keep track of what revisions and
posts have already been thanked for. This is problematic because client-
side storage is not permanent, making it easy to send duplicate thanks
if you have logged out and in, or switched computers.
This starts storing new thanks in the log_search table, which is
designed for efficient querying of metadata associated with a specific
log entry. With this, we can easily check to see if a user has already
sent thanks for a specific revision.
The UX is a bit weird right now, we only check log_search when actually
sending thanks again, in which case the user thinks they have sent
thanks again, but no duplicate thanks is actually sent.