RFC 145: Usage stats download button

Created by: tsenart

Fixes #10434

Next steps

• Test on Sourcegraph.com once it's deployed. Validate that it doesn't time-out.

Screenshot

@christinaforney: I don't know if it's worth it to add in product text about how long this operation may take. Perhaps it's enough to set expectations in the email as part of delivery plan. cc @ebrodymoore

Created by: tsenart

I want to still add an integration test that covers the HTTP handler, but I'd like a review already @unknwon.

Created by: tsenart

@unknwon: Please take another look.