Extend "Terms of service" element with the option to add a link

Very basic implementation which works well.

Needs some tests, and maybe something better as just a textfield for the url?

You can create a term of service with a link using a simple checkbox. I don't think this feature is needed.

terms:
  '#type': checkbox
  '#title': 'Terms of <a href="#link">service</a>'
  '#description': '<p><a href="#link">Here is a link to the terms of service in a description.</a></p>'
  '#required': true

Then why bother having a "Terms of service" field option in the first place? That's also just "a simple checkbox".

Let's see if the tests pass and we can merge this AS-IS.

Tests are not passing.