Admin Analytics: Allow user to download data

Created by: mike-r-mclaughlin

Requested by: https://github.com/sourcegraph/accounts/issues/3

Feature request description

Add a way for users to download data from the admin analytics pages.

Is your feature request related to a problem? If so, please describe.

Leaders of groups that have been onboarded would like to see usage data but they are not site administrators. Currently there is no way to give them access to this data. Downloading the data as a csv (or some other importable format) would allow these users access to the data and allow them to view the results in Excel or other tools.