Description

Closes #81. Implements the initial web step of #27 self-service registration.

• Adds anonymous /accounts/register GET/POST flow with NetID validation.
• Reuses UserStub creation and the existing emailed completion link.
• Uses RSVP-aligned card styling and mobile layout.
• Gives duplicate or failed requests safe, actionable responses.

Verification

• pipenv run python manage.py test accounts
• pipenv run python manage.py check
• Browser smoke: /accounts/register renders the heading, Net ID input, and submit button; invalid Net IDs display their error message.

Pre-Merge Checklist

• Focused tests and Django checks pass.
• Feature flag review: this public registration endpoint is intentionally exposed as the requested issue behavior.