Move: Differentiate between internal and external Move

[pfefferle]

To better handle different Move use cases, it makes sense to have a different Move function for internal (Move::internally( $from, $to )) and external (Move::externally( $from, $to )) moves.

To better handle the very specific internal Move command, this PR also introduces the Inherit Activity. This Activity allows developers to mark an Activity that would normally stored as Activity-Object, to be handles as the main Activity.

Proposed changes:

• Two different functions for internal moves (user-id changes, move abandoned users) and external moves (from WordPress to Mastodon).
• Implement Inherit Activity

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Go to '..'