Create a new page Admin - Settings - Service Usage Data

The first step to implement proposal: gitlab-org/gitlab#346372 (comment 810062175)

Problem

We're creating a separate page so users will be able to review, redact and submit usage ping.

On this stage we just want to let users to review UsageData payload and manual submit it. This issues covers only UI part.

Proposal

1. Create a new page Admin - Settings - Service Usage Data. This page will be under a Feature Flag
2. Move the existing UI element (“Preview Payload” button and related text area) to the new page. Rename the button to “View Payload” to "Download Service Ping". UI and interaction is implemented in plain JS (not Vue component). We should keep the existing behaviour (not to load payload with the page to reduce unnecessary and potentially heavy server request).
3. Reroute the GET /usage_data.html request to ApplicationSettingsController. I would suggest to leave moving this method to a new controller in future iterations when we have better idea regards the new page functionality.

Next steps (if any)

Implement backend part