Implement the view

Created by: sashaostrikov

Based on design, approved in https://github.com/sourcegraph/sourcegraph/issues/39997, create a UI for 1-click export.

Things to think about:

• ‼️ This page should be somewhere in site admin area
• There will be plenty of database tables to query, how to organise selecting multiple/all/none of them, so UI is not cluttered, yet functional
• This should be a quick thing for a user to use this feature, i.e. it should has some decent pre-selected set of things to export (which will suit most) along with convenient switches to enable/disable everything, to make it "2 clicks to export everything"

/cc @jplahn @ryphil