Group members Vue conversion - Wire up role dropdown with Vuex action

What does this MR do?

Part of &4233 (closed) and #235603 (closed)

As part of an effort to convert the group members page from HAML to Vue this MR wires up the Vuex actions added in !44254 (merged) with the role dropdown added in !44651 (merged)

⚠ Note: Currently changing the role for linked groups (in the "Groups" tab) does not work. This is because the API endpoint has not yet been passed to the Vue app. It will be setup in !45202 (diffs) as that MR also uses the path. ⚠

Currently this is behind the vue_group_members_list feature flag. To enable:

1. bin/rails console
2. Feature.enable(:vue_group_members_list)

Screenshots

Desktop

Mobile

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
  • Not needed, behind a 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