Restructured upload-by-url:

Restructured upload-by-url:
* In ApiUpload: moved stuff that is checking instead of actual uploading out of performUpload method
* Made UploadFromUrl conform to standards:
** In initialize* do only initialization, no actual work
** Moved file fetching to fetchFile
** Consistent use of tempnam()
** Perform the uploading in performUpload, don't define our own doUpload method
* Moved almost all job magic to the UploadFromUrlJob class. This way the job is almost a regular client, and we don't need many special cases to deal with async uploading.
* Made leaving a message optional; results will be stored in the session otherwise

I did not actually test the async uploading, because I first wanted to commit a properly working synchronous upload-by-url system.