Create REST endpoint which streams files for any type of project or document

The existing FileService doesn't support all document types for non-File projects, and it also relies on the client to choose suitable chunk sizes (to avoid oversized database BLOBs). We need a new API for files, which can stream large documents (wherever or however it stores them), and which performs any slow processing separately (to avoid HTTP timeouts)

For legacy projects, use the project type (xliff, properties, podir, etc) so that we can determine the full filename, with extension, for push and pull.

If the project type is NULL, return an error message which advises pushing the source files again with the latest client (so we can get the project type: ZNTA-1249).