R1:29a3cd5121d9

Add "Manual Activities", to tell administrators to rebuild the search index

Summary:
Ref T11922. After updating to HEAD of `master`, you need to manually rebuild the index. We don't do this during `bin/storage upgrade` because it can take a very long time (`secure.phabricator.com` took roughly an hour) and can happen while Phabricator is running.

However, if we don't warn users about this they'll just get a broken index unless they go read the changelog (or file an issue, then we tell them to go read the…