swift: use $upload.http for uploading files

This is to get support for upload progress. The ng-file-upload package
will monkeypatch the XMLHttpRequest a little so that it can get access
to the XHR created by $http. It needs access to it so it can attach an
onprogress event handler.

We're not currently using anything else from ng-file-upload.
upload-modal: show progress bars when uploading (fixes #76)
upload-modal: avoid reuploading files

This partially addresses #78: simply clicking the upload button after
uploading a file again will no longer do anything. However, it doesn't
fix the case where the user uploads a file named 'foo' to a container
already containing a file with that name.
upload-modal: disable upload button when all uploads are done
upload-modal: support removing files (fixes #79)