* Add hashtag filter to profiles

GET /@:username/tagged/:hashtag
GET /api/v1/accounts/:id/statuses?tagged=:hashtag

* Display featured hashtags on public profile

* Use separate model for featured tags

* Update featured hashtag counters on-write

* Limit featured tags to 10

Define `Settings::BaseController#set_body_classes` so that sub classes
inherit `Settings::BaseController` don't need to define
`#set_body_classes` agein.