Start transfer calls storage service

Updated Archivematica to call the storage service to move files when starting a transfer, instead of assuming all files are locally available.

Part of this work cherry-picked from dev/issue-6488-aip-reingest, specifically the work to upload metadata files from a transfer source.  This will need to be cleaned up if it is merged before AIP reingest is.

JS browser now tracks the UUID of the Location a path was selected from, and POSTs that to the dashboard in the form ```<UUID>:<full path>```.  Note the UUID is not displayed to the user - it is the 'id' of the element containing the path displayed to the user.

The dashboard POSTs to the storage service to move these to the pipeline's temp directory, then moves them to the correct watched directory when complete.

Some cleanup of dead code as well.
As long as we're updating these, switch to `os.path.join`?