Simplify download request popup design and behavior

The text displayed in a popup when a user requests the cooking of a tarball should be simplified.

Also once a user has submitted a request, a simple alert showing the request creation status and a link to the "Downloads'"page should be displayed instead of redirecting to the "Dowloads" page as it is currently the case.

See this proposed design and this one as references.

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Migrated from T2321 (view on Phabricator)