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