Per-Recipient notification silencing

This implements a first step towards per-recipient notifications as discussed on the mailing list and wished for in many tickets. So far, I only implemented the possibilities to silence certain people and groups permanently or temporarily, since this seemed to be the minimal consensus achieved on the mailing list.

**WARNING**: This upgrades your database. Although a downgrade should be comparatively easy (delete a table and downgrade the global database version), make sure to have a backup if you test this.