Refactor MLT XML conversion with proxies to just-in-time.

This frees the file.output_path for holding the rendered output, prevents
storing proxy XML files in transcodes, and allows for bug fixes to not require
rerunning validate jobs. Now, the MltXmlHelper::substituteProxyFiles() does
not write to the database and a similar new function checkFileReferences()
checks the state of dependencies updating the database as needed for
the validate job.
Add MLT XML rendering task to the encode job.
Upload new jQuery UI CSS and JS to prepare for user-metadata entry.
New jQuery UI fixes some small problems with buttons.
Fix and array index error in MltXmlWriter.
Clear the file status converting bit when done.
Fix handling the display of file status for project renders.