Consolidate some application email receiver code in preparation for API changes

Summary:
Ref T7477. The various "create a new X via email" applications (Paste, Differential, Maniphest, etc) all have a bunch of duplicate code.

The inheritance stack here is generally a little weird. Extend these from a shared parent to reduce the number of callsites I need to change when this API is adjusted for T7477.

Test Plan: Ran unit tests. This will get more thorough testing once more pieces are in place.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T7477

Differential Revision: https://secure.phabricator.com/D19950