Closes #9264.

Feature. Prevents registration form submission if any inputs are invalid -- covers taken username/screenname errors which would otherwise pass HTML checks and allow submission.

Technical

Adds an extra validity check (determining that no form inputs are invalid) before allowing form submission. This means that inputs that are invalid because they failed backend IA/OL availability checks, but passed HTML formatting tests, will still be caught and prevent form submission.

This strategy has the added advantage of ensuring any future JavaScript-based checks (i.e. if we move the email spamcheck into realtime_account_validation.js) will automatically prevent form submission, as long as they assign the input an invalid class.

As noted in the issue, this won't display helpful new error messages the way the HTML checks does, but further UI customization (i.e. highlighting existing error messages) could be included in the UI phase of #7694.

Also has a nice side effect of handling the edge case in where a user's browser doesn't support reportValidity -- if their submission has formatting issues, they won't see the nice HTML errors, but they still won't be able to submit.

Also added a few comments to explain what these checks do to prevent future developers removing seemingly duplicative checks.

Testing

1. Log out (if logged in)
2. Go to "Sign Up" or /account/create
3. Fill in all the inputs, ensuring that you pass all formatting checks, but that you enter a username or email that is taken on IA or OL -- feel free to use rebecca@rebeccashoptaw.dev or rstesting for testing purposes
4. The form should not submit

Screenshot

Stakeholders

@cdrini @mekarpeles