[#1703] Fix bug when uploading resources

The problem was that we used to set the URL input as "disabled", which caused
the browser not to send its value when submitting the FORM. The solution is
simple: use "readonly" instead.
[#1691] Update "Add group" explanation in "Adding a new dataset" docs
Merge pull request #1703 from ckan/1703-error-when-uploading-resource

Error when uploading new resource
Merge pull request #1691 from ckan/1691-update-add-group-docs

Remove "Add group" explanation on docs' "Adding a new dataset" page