An uncommon but somewhat difficult to digagnose issue is dealing with
improperly-seeded databases. In such cases, instance-signed fetches will
fail with a ActiveRecord::RecordNotFound error, usually caught and handled
as generic 404, leading people to think the remote resource itself has not
been found, while it's the local instance actor that does not exist.

This commit changes the code so that failure to find the instance actor
automatically creates a new one, so that improperly-seeded databases do
not cause any issue.

It's not useful for now, but it's required by ActivityPub

* 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