App setting to bypass do not disturb

[marbetschar]

trafficstars

Respects a user defined setting for a specific app to bypass do not disturb mode. Works together with the companion PR in the switchboard-plug-notifications repo: https://github.com/elementary/switchboard-plug-notifications/pull/84

[danirabbit]

I'm gonna bump this to OS 7 because I think we need more careful consideration around the design here than we have this month

[jeremypw]

This does work after restarting the notifications server.

[jeremypw]

@marbetschar Are you able to address the review comments - this PR is nearly mergeable?