Remove all hardcoded behaviors associated with task priorities

Summary:
Ref T3583. Currently, we have some hard-coded behaviors associated with the "Unbreak Now" and "Needs Triage" priorities. Remove them:

• Users seem somewhat confused by these on occasion, and never seem to think they're cool/useful (that I've seen, at least).
• I think they have low utility in general, see T3583.
• Saves three queries on the home page, which can no longer use row counting since they must be policy filtered.
• Primarily, this paves the way for allowing installs to customize priorities, which is an occasional request.

Also deletes a lot of code with no callsites.

Test Plan: Mostly grep. Loaded home page. Viewed reports and task list.

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T3583

Differential Revision: https://secure.phabricator.com/D6981