Cart widget appears when collections are added. It links to the Download tab

This pull request is the the first half of the requirements to show that the user has added some collections and to provide a link to Step 3

Next: add functionality to allow the removal of individual collections or clearing/reseting from the download cart.
I see.

It looks like the same problem has been fixed in a different branch (see 8e34fd743edcfd082401ffef8633e95e82d3a0bd#diff-886adee7d66465dd8c99f3e9db3157c0R55) and IMO that fix is in a better place (we actually know it's part of an onclick handler, there). Can we get rid of this `return false` if that one fixes it?