Develop single- and multi-select plugins with feedback

MR submitted with new plugins `select_single_feedback` and `select_multiple_feedback`.

Left comments on the MR, so far so good!

@graber thanks for the feedback, will update!

Updated MR with feedback incorporated.

Needs a bit more work.

Thanks for the excellent feedback. MR updated.

Made a couple more minor tweaks.

Just a tiny cleanup and ensured the activity-specific LMS selector is used in the single choice plugin.
Thanks!

Automatically closed - issue fixed for 2 weeks with no activity.