Fixed creation of new image

Removed creation of template from url/uploaded file from base image model.
Created DownloadService for downloading files from url.
Replaced improt_from_xml by DownloadService in existing code

There is still an obsolete import_xml_from_url method in
app/util/deployable_xml.rb which is apparently used only in tests.
This will be removed in separate patch because it fix is not
straightforward.