Implementation of ghost banning for users

Users can now be ghost banned via controls on their profile accessible to admins (see screenshot below)

** this pull request has migrations **

* When a user is banned, all of their posts are removed from search index, thus removing them from Fresh, Trending, Popular protip pages.
* When a banned user creates a new protip, that tip is not indexed in search.
* A banned user can still see their protips from their tip listing page.
* A banned users' protips are still available at their respective #show pages.
* An admin can unban a user if needed from the profile page.
* If a user is unbanned, their protips are reindexed for search.

![ban_control](https://cloud.githubusercontent.com/assets/197892/3447056/295b58ca-0149-11e4-8bd0-ba468801150b.png)