Create Service Account Group and Instance Rails Controller/Route/Feature Flag and Navigation

We surface service accounts at the instance and top level group.

This issue covers the following work:

• Re-use feature flag :service_accounts_crud and remove old usages of it.
• Create rails route and rails controller for index action for top-level service level account and instance level service account.
• Render blank page if flag enabled, if flag disabled, render 404
• Add feature flagged show/hide logic for top-level group and instance level nav items (see screenshots)
• rspec controller unit tests.

We will bootstrap two difference instances of the service accounts Vue app. One in the instance level context, and one at the top-level group context.

Screenshots

• Note the screenshots are only providing the examples for the nav. The page contents once you click on the nav will be in a follow up issue. We will render a blank page for this initial issue.