Show last ping data to admins on the Site-admin > Pings page

Created by: dadlerj

https://app.hubspot.com/contacts/2762526/company/557692623 requires offline support for Sourcegraph, and we have agreed that they will simply email us a copy of the standard telemetry, but once per month (or on some other schedule).

This dovetails nicely with the desire to be more transparent about our ping telemetry. We have discussed in the past displaying the actual telemetry payloads to admins for full transparency, and that could solve this problem.

Proposal

• On the Site-admin > Pings page, add a box at the top (or bottom? or on another tab?) that illustrates the payload generated for the "last" ping (or if not exactly the last, some ping in the last 24 hours) that was sent.
• Add a button to the top to download that payload (similar to the download button on https://sourcegraph.com/site-admin/usage-statistics).
• Ensure that this whole flow works, even if the instance is not connected to the internet (so pings aren't actually going out). In this case, I believe we still generate the payload, but the post to sourcegraph.com fails, so just make sure everything is sequenced correctly...