Group members Vue conversion - approve access request button

What does this MR do?

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

I am working on converting the group members page from HAML to Vue. This MR adds the approve access request button to the "Access requests" tab.

Local testing

Currently this is behind the vue_group_members_list feature flag.

1. bin/rails console
2. Feature.enable(:vue_group_members_list)
3. Create a group
4. Navigate to /admin/users and choose a user
5. Click the "Impersonate" button in the upper right corner
6. Navigate to the newly created group "Overview" page and click the "Request access" link next to the group name
7. Stop impersonation in the upper right corner
8. Navigate to "Group" -> "Members" and click the "Access requests" tab

⚠ You will not see the approve access request button because this MR relies on permissions.canUpdate which is being implemented in !44651 (diffs). If you would like to test locally you can temporarily remove v-if="permissions.canUpdate" from !44676 (diffs). There are tests for this so we can be confident that the functionality is working as expected. ⚠

Screenshots

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