Expose all application mail receivers
Summary:
Fixes T7199. This still isn't a shining example of perfect code, but the raw amount of copy/paste is much lower than it used to be.
- Reduce code duplication between existing receivers.
- Expose receiving objects in help menus where appropriate.
- Connect some "TODO" receivers.
Test Plan:
- Sent mail to every supported object type.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T7199
Differential Revision: https://secure.phabricator.com/D12249