R1:dc7d0b4a56d8

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…