Add admin users vue app wrapper

What does this MR do?

Part of Migrate admin/users to Vue MVP &4961 (closed).

This adds a Vue app wrapper to admin/users as a first step to converting the page from HAML to Vue.

For the MVP we're passing the data from HAML to Vue via data-attributes, once this MR is merged work can begin to migrate to GraphQL #290276.

Local Testing & Setup

1. Enable the development feature flag in rails console Feature.enable(:vue_admin_users)
2. Visit /admin/users and check that the Vue app is showing in the Vue console with the correct props

Screenshots (strongly suggested)

Can only be viewed using Vue Dev Tools

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry - Behind a feature flag
• [-] Documentation (if required) - No visual changes
• 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 #285104 (closed)