* Fix #2672 - Connect signed PuSH subscription requests to instance domain

Resolves #2739

* Fix return of locate_subscription

* Fix tests

* Move ApiController to Api/BaseController

* API controllers inherit from Api::BaseController

* Add coverage for various error cases in api/base controller

* Coverage for api push controller

* Refactor the api/push controller

* Fix #2119 - Whenever about to send a HTTP request, normalize the URI

* Add test for IDN request in FetchLinkCardService

* Perform IDN normalization on domains before they are stored in the DB

* Allow running mastodon on a different domain as the one used for identifying users

* Alter documentation of WEB_DOMAIN to make clear it shouldn't be used unless the admin knows what they are doing

* Compare to web_domain instead of local_domain when dealing with feeds/API

* Correctly identify mentions to local accounts

Mentions URLs point to the person's web profile, i.e., the user page served on WEB_DOMAIN.