Tie application event listeners to the applications they listen for

Summary:
Ref T3675. Some of these listeners shouldn't do their thing if the viewer doesn't have access to an application (for example, users without access to Differential should not be able to "Edit Tasks"). Set the stage for that:

• Introduce PhabricatorEventListener, which has an application.
• Populate this for event listeners installed by applications.
• Rename the "PeopleMenu" listeners to "ActionMenu" listeners, which better describes their modern behavior.

This doesn't actually change any behaviors.

Test Plan: Viewed Maniphest, Differntial, People.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3675

Differential Revision: https://secure.phabricator.com/D7364