Feature: Customise notifications per-inbox on a global and per-user basis

[Jonty]

We need to be able to customise notifications for all users, and then allow per-user overrides per-inbox.

This needs to look similar to how Helpmonks did it. Currently each user just has a global setting for notification types, and then a per-inbox mute.

[Jonty]

Note: We currently have a patch that simply turns off notifications to other users of an inbox when we send an email, but this should clearly be a setting.