Change unapproved and unconfirmed account to not be accessible in the REST API (...

Change unapproved and unconfirmed account to not be accessible in the REST API (#17530)

* Change unapproved and unconfirmed account to not be accessible in the REST API

* Change Account#searchable? to reject unconfirmed and unapproved users

* Disable search for unapproved and unconfirmed users in Account.search_for

* Disable search for unapproved and unconfirmed users in Account.advanced_search_for

* Remove unconfirmed and unapproved accounts from Account.searchable scope

* Prevent mentions to unapproved/unconfirmed accounts

* Fix some old tests for Account.advanced_search_for

* Add some Account.advanced_search_for tests for existing behaviors

* Add some tests for Account.search_for

* Add Account.advanced_search_for tests unconfirmed and unapproved accounts

* Add Account.searchable tests

* Fix Account.without_unapproved scope potentially messing with previously-applied scopes

* Allow lookup of unconfirmed/unapproved accounts through /api/v1/accounts/lookup

This is so that the API can still be used to check whether an username is free
to use.