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.

Migrated from T2321 (view on Phabricator)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information