The old "sync pr" thing is turning out to be a bust, while it
originally worked fine these days it's a catastrophe as the v4 API
performances seem to have significantly degraded, to the point that
fetching all 15k PRs by pages of 100 simply blows up after a few
hundreds/thousands.
Instead, add a table of PRs to sync: if we get notified of a
"compatible" PR (enabled repo & target) which we don't know of, create
an entry in a "fetch jobs" table, then a cron will handle fetching the
PR then fetching/applying all relevant metadata (statuses,
review-comments and reviews).
Also change indexation of Commit(sha) and PR(head) to hash, as btree
indexes are not really sensible for such content (the ordering is
unhelpful and the index locality is awful by design/definition).
