Migrate sign in form to Vue

Implementation guide

See !206825 (closed) for some of the starting point

1. Render a simple HAML form. This will help avoid page jump and will be replaced with Vue from when rendered.
2. Use parseRailsFormFields to get input names and values
3. Render a Vue form using GlFormFields
4. Validation should only happen on submit. This will be made configurable in gitlab-org/gitlab-services/design.gitlab.com#3038 (closed)
5. Ensure form still works with password managers

TODO

• Implement initial sign in form - !209857
• Make sure logic in https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/pages/sessions/new/index.js works with Vue form. Move logic to Vue if needed
• Manually test all flows authentication flows
• Test popular password managers
• Gradually enable feature flag