Moved (& improved) all validation logic out of views, and into forms.

Additional validation logic includes validating that the exercise_ids and youtube_ids POSTed to save_*_log are actually recognized--otherwise, report the validation error to the user.

Note that this stricter checking would make KA Lite uncustomizable, without modifying the NODE_CACHE and other json objects.  So it's worth considering whether this stricter check is what we really want.