Move invite teammates code to shared area

What does this MR do?

• Moves previously registration/groups only code to a shared area for reuse during https://gitlab.com/gitlab-org/gitlab/-/issues/219241 via !48794 (merged)
• Defaults the invite email to one blank input to align with our design with our use in https://gitlab.com/gitlab-org/gitlab/-/issues/219241 as well. (same look and feel)
• Adds link for a pointer to docs on how to change roles.
• Changes wording from teammates to Members to align with inviting in other areas of the application.

Note

1. These changes are behind a feature flag, hence no changelog.

2. In order to see these changes locally, enable the onboarding_issues experiment in the console by running:

   Feature.enable_percentage_of_time(:onboarding_issues_experiment_percentage, 100)

3. Most of the code is a move of code implemented during !38169 (merged) and !37953 (merged)

Sign in and visit: http://localhost:3000/users/sign_up/groups/new

Screenshots (strongly suggested)

platform	before	after
desktop
mobile

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

Related to #219241