Pass group member data from HAML to Vue

What does this MR do?

Part of &4233 (closed)

This is the first step in converting the group members pages from HAML to Vue to improve UX/UI and move towards Pajamas components.

As a MVC we will pass the data from HAML to Vue via data attributes and props. The idea is to set this up so down the road it could be switched to use GraphQL.

Screenshots

No changes to UI yet

Local Testing

This MR does not render anything in the UI yet, but you can use Vue Dev Tools to verify that the data has been correctly passed from HAML to Vue.

1. Enable vue_group_members_list feature flag
   • bin/rails console
   • Feature.enable(:vue_group_members_list)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
  • Not needed, behind feature flag
• [-] 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