Migrate sign in form to Vue

Implementation guide

See !206825 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
5. Ensure form still works with password managers