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

resources/sql/autopatches/20160218.callsigns.1.sql

@@ -0,0 +1,4 @@
+/* Make callsigns nullable, and thus optional. */
+
+ALTER TABLE {$NAMESPACE}_repository.repository
+  CHANGE callsign callsign VARCHAR(32) COLLATE {$COLLATE_SORT};