disabled evaluation submit button after submission is made.

It has been reported that students are getting confused with the
warning message saying that the answer pair they have just evaluated
has already been evaluated before; therefore the submission failed.
However, what really happened, is that the student submitted multiple
times, so the first submission was successful and any subsequent submission
caused the error/warning messages.

Now, I have implemented the "submitted" boolean to prevent this problem
from happening. This implementation is actually done for multiple forms
in the application.
removed the the brief moment where the submit button re-enables.