Add social sign up on new sign up flow

What does this MR do?

Part of gitlab-org/growth/engineering#98 (closed)

Social sign up buttons where missing in the experimental sign up flow. This MR adds them again with slight design changes. Since Google and GitHub are the most used sign in options, we prioritise these now. This is the first iteration. Next step will be to not display all options and add the "More options" part in the design.

Screenshots

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

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