Improve the flag.* Conduit methods

Summary:

• Put the code to generate informational dicts about flags into the base class.
• Update flag.delete to accept an object PHID in order to delete the flag on that object, since currently the model is that each object may have at most one flag, and each flag has exactly one object, although the former is not enforced.
• Add flag.edit, which creates or updates a flag, optionally with the given color and note.

Test Plan:
Spend endless hours repeatedly running arc call-conduit and
arc flag.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3141