refac(submit): extract into a handler

[abhinav]

Building upon the prior two changes,
this refactors the submit commands to all be backed
by the same SubmitHandler implementation.

This allows completely dropping the "submit session" concept
as its only purpose was to pass information between "*stack submit"
and "branch submit" commands.

[abhinav]

This change is part of the following stack:

• refac(branch track): extract into a Handler #739
  • refac(branch checkout): extract checkout logic into a handler #740
    • refac(submit): extract into a handler #741 ◀
      • refac(branch delete): extract handler #742

_{Change managed by git-spice.}