Check required fields for video upload form.

Actually I didn't know it is so intrusive to add `class="required"` for Django Forms Fields.

For now I generate a ruleset (jquery-validation plugin  http://docs.jquery.com/Plugins/Validation/rules) to check required fields.

A more powerful approach might be: https://github.com/agiliq/Django-parsley & http://parsleyjs.org/ 

Don't forget to `python manage.py collectstatic`

Is `linkURL` really a field that must be in this form?