* Make embeds cacheable by reverse proxy

* Make follow button on embeds open remote follow modal

Instead of web+mastodon://, also, turn the button blue, and add a
sign up prompt to the remote follow modal

* Add coverage for create with empty acct value

* Add coverage for create with webfinger failure

* Add coverage for create with webfinger providing bad values

* Add coverage for create when webfinger is good

* Add coverage for session[:remote_follow] having data

* Simplify how remote follow pulls acct from session

* Remote follow behaves more like model

* Move the discovery portions of remote follow out of controller

* Check for suspended accounts

* Consolidate webfinger string creation under Account#to_webfinger_s

* Introduce Account#local_username_and_domain for consolidation

This stores the @username@instance you provide in your session and
reuses it the next time you remote follow someone from this instance.