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.
But I was young and naive, Phil. ;)

I won't wait for this to do the merge, Phil. If we want to go through with it we can add it after.