Learner Groups Fixes

When adding a user in the learner groups make sure we only the current group cohort. This allows them to be used in any other groups as well without refreshing the page.

Cleaned up group selection elements, that had some commented out code from early ilios beta. Since groups can not be selected in the picker all of this can be pulled out.

Added tests for various learner group selection scenarios, hopefully covering everything that this fix might break.

Fixes #199