Add a defaulty priority configuration flag for maniphest tasks

Summary: instance-wide this setting be

Test Plan: made a new task and noted the default priority honored what was in btrahan.conf

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1842

Differential Revision: https://secure.phabricator.com/D3626