Apply standard registration flow to single-sign-on sign-ups

What does this MR do and why?

Resolves #339256

• Changes the single-sign on registration flow to match the standard registration flow.
• This change only affects the registration users/sign_up page. If a new user single-sign's on from the sign-in page users/sign-in then they get taken to the dashboard.

Screenshots or screen recordings

oauth-registration-flow

How to set up and validate locally

1. Create an OAuth app in Github
2. Add Github as an omniauth provider in config/gitlab.yml

development:
  <<: *base
  omniauth:
    allow_single_sign_on: ['github']
    block_auto_created_users: false
    providers:
      - { name: 'github',
          app_id: 'YOUR_APP_ID',
          app_secret: 'YOUR_APP_SECRET',
          url: "https://github.com/",
          verify_ssl: false,
          args: { scope: 'user:email' } }

3. Enable the feature flag

$ bundle exec rails c
Feature.enable(:update_oauth_registration_flow)

4. If you want to retest the flow multiple times, you can delete the records from the following tables:

$ user_id = User.find_by(email: 'YOUR_OAUTH_EMAIL').id
$ Identity.find_by(user_id: user_id).delete
$ User.find(user_id).delete

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.