Add some indexes to speed up queries

This PR will add some indexes to our database that should MASSIVELY improve query time in some cases.

(who_id, whom_id) on followers should improve deletion on followers, one of the queries we spend most time on currently.
(created_at) should drastically improve sorting time on messages (on fx. the /public page). Tested on production-sized database and its something like a factor of 100.
(flagged) is mostly just for good measure since we often filter on it. Since we don't really use it I don't know how much this will really improve but no reason not to

git-gurus-itu-devops / itu-minitwit