R1:20e6a4200dbc

Validate configuration of `maniphest.priorities`

Summary: Fixes T6132. We currently allow invalid configuration here; validate it.

Test Plan: Tried to save invalid config (negative priorities, string priorities, etc).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6132

Differential Revision: https://secure.phabricator.com/D14689