Don't create notifications from ignored people

Also move "shareable hidden"-logic to AlsoCommented, because it is the
only one that needs it. And write some specs for mentioned and started
sharing notifications.

Fixes #6294

Display markdown images inline

Update gems

otherwise we have data-inconsistency if the user stops ignoring the
person.

[API] don't store ID tokens in DB

fix #6857

fix migration for invitation-cleanup