Whenever an actor makes an activity (like, announce, etc.) we now save it in our db, so the url we pass to other activity pub servers is valid and can return the correct json.
All the factory and wrapper classes now return an `Activity` entity that can be converted to json using the new `ActivityJsonBuilder`

# Conflicts:
#	src/MessageHandler/ActivityPub/Inbox/FollowHandler.php
#	src/MessageHandler/ActivityPub/Outbox/AnnounceLikeHandler.php
#	src/MessageHandler/ActivityPub/Outbox/FollowHandler.php

� Conflicts:
�	composer.lock
�	tests/WebTestCase.php

� Conflicts:
�	composer.lock

- Remove the static nature of the `SettingsManager` and the `MentionManager`
  - If the environment is not in `test` then the `SettingsManager` will still use a static cache, but all static methods were replace with instance methods
  - All static methods from the `MentionManager` were replaced with instance methods
- Remove some Doctrine cascade removes (removing a message thread with the entity manager resulted in all participating users being purged)
- Handle pin announces in the message handler themselves instead of the pin event
- pass the incoming activities through to the messages in specific cases
- create `Activity` entities for remote payloads under specific circumstances (so we can announce the original payload)
- improve webfinger exception message
- Add functional ActivityPub testing. This is very complicated and requires a lot of code. Sadly it is also not thread-safe for a reason I could not find out, yet. In every setup method we create actors for different domains, so we can generate some activities, webfinger jsons, and everything we need to mock the activities needed to test the current part of the AP code. Each message handler is tested in its own test class