Make repository callsigns optional

Summary:
Ref T4245. This could still use a little UI smoothing, but:

• Don't require a callsign on the create flow (you can add one later in "Edit Basic Information" if you want).
• Allow existing callsigns to be removed.

Test Plan:

• Created a new repository with no callsign.
• Cloned it; pushed to it.
• Browsed around Diffusion a bunch.
• Visited a commit URI.
• Added a callsign to it.
• Removed the callsign again.
• Referenced it with R22 in remarkup.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D15305