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