MDL-38128 mod_assign: Adding group selector to grading summary page

Thanks to Pau Ferrer Ocaña and Avi Levy for
their work on this issue.
MDL-38128 mod_assign: Fixing count teams and submissions with status

We should restrict the groups to the activity active group. Also
improving performance caching get_submission_group() results.
MDL-38128 mod_assign: Counts and groups unit tests