Only ask for branch to deploy if current branch is not master.
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.
Fix handling of extra markup for user partial
Display number of followers
Update '.gitignore'
Change 'config/deploy/production.rb'
Remove appsignal stuff
Don't use S3 datastore
Deploy from fork repo
Set correct Ruby version
Don't use chruby
Use 'capistrano/rbenv'
Fix dotenv config

We don't use 'dotenv-deployment' for production because it doesn't work
for our setup.
Remove turbolinks crap
Fix `deploy:restart` capistrano task
Don't display "Login" link
Put 'quiet_assets' gem into `:development` group
Put 'puma' gem into `:staging, :production` group
Remove 'rack-cache' gem
Load fonts from the very same host