* Change account and user fabricators to simplify and improve tests

- `Fabricate(:account)` implicitly fabricates an associated `user` if
  no `domain` attribute is given (an account with `domain: nil` is
  considered a local account, but no user record was created), unless
  `user: nil` is passed
- `Fabricate(:account, user: Fabricate(:user))` should still be possible
  but is discouraged.

* Fix and refactor tests

- avoid passing unneeded attributes to `Fabricate(:user)` or
  `Fabricate(:account)`
- avoid embedding `Fabricate(:user)` into a `Fabricate(:account)` or the other
  way around
- prefer `Fabricate(:user, account_attributes: …)` to
  `Fabricate(:user, account: Fabricate(:account, …)`
- also, some tests were using remote accounts with local user records, which is
  not representative of production code.

* Change silenced accounts to require approval on follow

* Also require approval for follows by people explicitly muted by target accounts

* Do not auto-accept silenced or muted accounts when switching from locked to unlocked

* Add `follow_requests_count` to verify_credentials

* Show “Follow requests” menu item if needed even if account is locked

* Add tests

* Correctly reflect that follow requests weren't auto-accepted when local account is silenced

* Accept follow requests from user-muted accounts to avoid leaking mutes