Add federation relay support (#7998)

* Add federation relay support

* Add admin UI for managing relays

* Include actor on relay-related activities

* Fix i18n

config/locales/simple_form.en.yml

@@ -13,6 +13,7 @@ en:
           other: <span class="name-counter">%{count}</span> characters left
         fields: You can have up to 4 items displayed as a table on your profile
         header: PNG, GIF or JPG. At most 2MB. Will be downscaled to 700x335px
+        inbox_url: Copy the URL from the frontpage of the relay you want to use
         irreversible: Filtered toots will disappear irreversibly, even if filter is later removed
         locale: The language of the user interface, e-mails and push notifications
         locked: Requires you to manually approve followers
@@ -52,6 +53,7 @@ en:
         expires_in: Expire after
         fields: Profile metadata
         header: Header
+        inbox_url: URL of the relay inbox
         irreversible: Drop instead of hide
         locale: Interface language
         locked: Lock account