Implement Client-Side Submission Prevention for Registration Form

[rebecca-shoptaw]

Dependency to epic #7694.

Describe the problem that you'd like solved

While the real-time validation fixes in #2055 will provide helpful error messages to the user re: bad formatting or taken emails, etc., they do not prevent the user from submitting a form with bad data.

For instance, if a user sees a warning that their username is too short, they will now get a nice on-screen warning, but if they choose to submit anyway, they will be able to, and duplicated and inefficient validation will happen on the backend (via forms.py) .

Instead, we should use a combination of HTML and JavaScript to prevent a user from submitting any form that contains bad or missing inputs.

Requirements

• [ ] #9244
  • PR: #9245
• [ ] #9264
  • PR: #9267

Additional context

Stakeholders

@cdrini