submit: support --publish opt-in mode

[abhinav]

Change --no-publish to --publish, defaulting to true, with a --no-publish opt-out. Support setting the default value via configuration, allowing users to always opt out of creating CRs, unless the --publish flag is added to the command.

This will make the experience of using submit with unsupported forges smoother.

[abhinav]

This change is part of the following stack:

• 355 ◀

_{Change managed by git-spice.}