Add option for importing a course from a named branch of a git repo

This adds a form field to the course tab of the sysadmin dashboard that can be used to optionally specify the branch name of the course repository you wish to add.  It also adds branch as an additional argument to the git_add_course management command.  I also changed the name of the platform version setting from the old MITX to EDX_PLATFORM while I was in there.

Suggested reviewers:
@cpennington @singingwolfboy @nedbat

Heads up to @sarina that this should be our last one for the semester push, and we realize it may be a bit late getting in.