R1:092f540199b7

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.