Assignment form validation

This handles the validation of submission rules (https://github.com/MarkUsProject/Markus/issues/1621) by attempting to fix the issue of Rails not validating the form and showing error messages (and instead just crashing).

Only problem is that the `render` calls don't seem to work, but `redirect_to` does, which causes the previously entered data to be lost (but the flash error messages work!).

The previously added client-side validation (disable submit button until mandatory fields filled out) is also dropped in this PR since the goal is to get the Rails validation to work.