Ensure import invoked from lms sets the branch to draft

LMS-6643

@carsongee this should fix the problem you found. Can you verify please? (it worked for me, but mileage varies)
@nedbat can you review?

@nasthagiri and I will do a more thorough fix to the unit tests and possibly move the `set_branch_setting` to a method on `ModuleStoreWriteBase` later. I wanted to get out a quick fix for MIT.

I'm going to look through the other lms management commands and put together a PR for any changes needed for those before any other work.