Add Actor migration

[pfefferle]

This adds the fields that are required for the ActivityPub target domain.

Changes proposed in this Pull Request

• https://docs.joinmastodon.org/user/moving/
• https://blog.joinmastodon.org/2019/06/how-to-migrate-from-one-server-to-another/

[pfefferle]

Hey @jessLundie, this is the only change we have to add to the target site.

I am now preparing the extension for the old blog, that triggers the move and adds the movedTo. I decided to not build that directly into the plugin (yet), because it's harder than I thought to have a generic solution.