Fix a bad variable access in rbt review edit.
Fix a bad variable access in rbt review edit.

`rbt review edit` was attempting to access `options.no_ship_it`, which
doesn't exist and caused a crash. Both `--ship-it` and `--no-ship-it`
set `options.ship_it`.

This has been reworked to check just `options.ship_it`, and to check for
its presence using `None` rather than truthiness/falsiness.

Testing Done:
Tested all `rbt review` commands and all options, verifying the results
on my test server.

Reviewed at https://reviews.reviewboard.org/r/12187/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools