Make Maniphest task priorites user-customizable

Summary: Drive these purely out of configuration after removing behavioral hardcodes in D6981.

Test Plan:
Mucked around with them:

{F58128} {F58129} {F58130}

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran

Differential Revision: https://secure.phabricator.com/D6984