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.
I'd not change these because the `presence: true` style is actually the new recommended style since Rails 3.