Require "URL" on self-organized request form

For the [self-organized workshop form](https://amy.carpentries.org/forms/self-organised/), the workshop url should be a required field. This does not apply to the workshop request or workshop inquiry form.

Start from the linked self-organized workshop form and locate its implementation in the Django application. Make the workshop URL required there, then verify that the workshop request and workshop inquiry forms remain unchanged.

Written by the indexing model from the issue text.