Download cart remodel

This PR "finishes off" the few remaining things that were needed to get end-to-end download cart/layer store integration, namely:

* download comes back as an actual download (instead of in an ajax response)
* removing layer from map removes associated record from the download cart
* only links considered to be "downloadable" are actually downloaded (a geonetwork record can contain other link types which we don't want to attempt to download and package)