Add "short name", "id", and "phid" variables for external editor URIs

Summary: Ref T13515. External editor URIs currently depend on repositories having callsigns, but callsigns are no longer required. Add some variables to support configuring this feature for repositories that do not have callsigns.

Test Plan: Changed settings to use new variables, saw links generate appropriately.

Maniphest Tasks: T13515

Differential Revision: https://secure.phabricator.com/D21147