closes #7546

closes #7545

closes #7542

Fixes #7541

This commit introduces support for AccountMigration federation message
receive. It covers the cases when the new home pod for a user is remote
respective to the recepient pod of the message. It also allows to initiate
migration locally by a podmin from the rails console. This will give the
pods a possibility to understand the account migration event on the
federation level and thus future version which will implement migration
will be backward compatible with the pods starting from this commit.

Add explicit mutual contact creation to the DataGenerator#generic_user_data
which is used in account deletion tests.

This doesn't add any value in the post preview:
* Avatar is already at the left-hand side of the publisher
* Timestamp is always "less than a minute ago"
* Interactions are disabled in the preview anyway

We don't display these things in the comment preview either, they only
need space and don't add any value to the preview.

Otherwise the export stays soft-locked and the podmin needs to reset it
manually.

Fixes #6225

Also remove `disconnect_contacts` methods, because contacts are already
removed with aspects memberships in `before_destroy`.

* load records to array to check them
* test that only 15 people are returned

* don't use `double` for queries
* use `second` instead of `at()`.

Or remove it where not needed

Also remove some unused html responses