R1:358baf1b3a18

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…