Introduce admin_groups_vue feature flag

What does this MR do and why?

Related to #541527 (closed) and &17783 (closed)

We are starting to migrate Admin area -> Groups from HAML/vanilla JS to Vue components. This MR introduces the feature flag for this migration but does not make any visual changes. In future MRs we will set up the Vue app, GraphQL query, etc.

References

Screenshots or screen recordings

No visual changes, exiting view still works (see below)

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