Make callsigns editable on repository basic information

Summary:
Ref T4245. This is a prelude to removing them from the "create" screen.

Currently, if you try to delete the callsign you get an unceremonious database error, but the next diff (or maybe two) will permit that, so I didn't put any "this is required yada yada" text in.

This could also maybe use some big flashing warning lights and a "if you edit this, all your URIs break" but I'll save that for later.

Test Plan: Changed the callsign for a repository.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D15304