* 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.

* Fix issues with POSIX::Spawn, Terrapin and Ruby 3.0

Also improve the Terrapin monkey-patch for the stderr/stdout issue.

* Fix keyword argument handling throughout the codebase

* Monkey-patch Paperclip to fix keyword arguments handling in validators

* Change validation_extensions to please CodeClimate

* Bump microformats from 4.2.1 to 4.3.1

* Allow Ruby 3.0

* Add Ruby 3.0 test target to CircleCI

* Add test for admin dashboard warnings

* Fix admin dashboard warnings on Ruby 3.0

- Reduce time-to-digest from 20 to 7 days
- Fetch mentions starting from +1 day since last login
- Fix case when last login is more recent than last e-mail
- Do not render all mentions, only 40, but show number in subject
- Do not send digest to moved accounts
- Do send digest to silenced accounts

Some available languages lack translations for notification mails. Now it
tests for two languages which is certain to have required translations:
German and English.

German is the language the current project owner, Eugen Rochko speaks, and
providing English translations for new messages is de facto mandatory.

* Localize date in digest

* Cover NotificationMailer more

* Dont use raise_error by itself (avoids warning)

* Add coverage for AccountFilter

* Improve coverage and refactor for Subscription#lease_seconds

* Improve coverage and refactor for NotificationMailer

* Simplify assignment of min/max threshold on subscription

A couple of places were using "reblog" rather than "boost" - this updates them to match the web UI