mbsubmit: Allow submitting release after initial import

Thanks @doronbehar, for the recent additions to the mbsubmit plugin #4807. Currently, the plugin only allows submission only on initial import. We should also extend the `mbsubmit` command to submit the release after the initial import.

Specifically, in addition to the track listing, `beet mbsubmit album:"XYZ"` should also give an option to `Open files with Picard`.