Now preview state can be one of (allowed, cancelled, empty-in-edit).
In case of editing a message without preview we set the state to
empty-in-edit and it changes to allowed if the links in the message
are changed somehow.
That way we don't need to cancel the preview when editing a message
with a cancelled preview and at the same time adding a link to
a message that had no preview in the first place will add a preview.
