Add a priorityColor property to the maniphest conduit endpoint

Summary:
I added a getTaskPriorityColor function to the ManiphestTaskPriority class which returns the color set in the maniphest config for the given priority.

This is in preparation for a change to arcanist which will allow it to display the priority color (if it is a supported color) upon running `arc tasks`.

Fixed some linting issues

Test Plan:
Invoke the maniphest.info method from conduit and ensure that:
 * The priorityColor property is given in the json
 * the priorityColor property is set correctly

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8734