Split sign in and sign up pages

What does this MR do?

Part of gitlab-org/growth/engineering#64 (closed)

Instead of using tabs to switch between the sign and sign up pages, we wanna have two different pages. This is part of an experiment that was already rolled out on gitlab.com.

This does not remove the signup_experiment completely yet since it's still used to determine if "confirmation" should be skipped for sign up which I would like to tackle in a separate MR.

Screenshots

Sign in

Before	After	After with LDAP

Sign up

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team