Don't read full image media into RAM on copying (#419)

Don't read full image media into RAM on copying (#419)

We copy uploaded media from the queue store to the local workbench
and then to its final destination. The latter was done by simply:
dst.write(src.read()) which is of course evil as it reads the whole
file content into RAM. Which *might* arguably still be OK for
images, but you never know.

Make use of the provided storage() methods that offer chunked copying
rather than opening and fudging with files ourselves.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>