Add ActivityPub actor representing the entire server (#11321)

* Add support for an instance actor

* Skip username validation for local Application accounts

* Add migration script to create instance actor

* Make Codeclimate happy

* Switch to id -99 for instance actor

* Remove unused `icon` and `image` attributes from instance actor

* Use if/elsif/else instead of return + ternary operator

* Add instance actor to fresh installs

* Use instance actor as instance representative

Use instance actor for forwarding reports, relay operations, and spam
auto-reporting.

* Seed database in test environment

* Fix single-user mode

* Fix tests

* Fix specs to accomodate for an extra `Account`

* Auto-reject follows on instance actor

Following an instance actor might make sense, but we are not handling that
right now, so auto-reject.

* Fix webfinger lookup and serialization for instance actor

* Rename instance actor

* Make it clear in the HTML view that the instance actor should not be blocked

* Raise cache time for instance actor as there's no dynamic content

* Re-use /about/more with a flash message for instance actor profile

config/locales/en.yml

@@ -24,6 +24,9 @@ en:
     generic_description: "%{domain} is one server in the network"
     get_apps: Try a mobile app
     hosted_on: Mastodon hosted on %{domain}
+    instance_actor_flash: |
+      This account is a virtual actor used to represent the server itself and not any individual user.
+      It is used for federation purposes and should not be blocked unless you want to block the whole instance, in which case you should use a domain block.
     learn_more: Learn more
     privacy_policy: Privacy policy
     see_whats_happening: See what's happening