capa: submit/final submit button naming

We have found that users were getting confued by the buttons named
"check," and on their last allowed attempt, "final check."  Check looks
implies something for the user only, not a real problem submission.

So this changes the wording to "Submit" for most attempts, and "Final
Submit" for the last attempt allowed.

We (Stanford) had made a prior change to only say "Submit" on the final
attempt, but condisering that now that seems even more confusing.