closes #7542

Fixes #7541

Account migration model/message

added tag for supporting apps prior to Chrome M39

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.

updates tag to follow standard styleguide

Re-add disconnect_contacts in AccountDeleter

Update thor, coveralls and simplecov

Remove avatar, name, timestamp and interactions from publisher preview

Reset stuck exports and handle errors

It looks like all commands we use in production fixed the warnings
(rails 5.1), so we can update that again.

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

Add missing indexes from #7234

Update gems

See #7234

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

Render mentions in comment previews correctly