Track remote followers. (#13)

* Adds `User#remote?`, reverse of `User#hosted?`
* `UserFetcher` now never fetches remote users.
* Adds `FriendshipChecker` and `RelFollowingChecker` service classes.
* Makes `WebmentionHandler` defer to `RelFollowingChecker` if target is user's homepage.
* Simplify `Webmentioner`.
* Lists known followers (including remote) on "Following" page.
* Periodically runs `FriendshipChecker`, which checks for a rel-following for remote friendships every 6 hours or so.