Create OC_JSON class, for single point of creating json responses.
No real logic change, this just cleans up the code a bit.

Signed-off-by: Florian Pritz <bluewind@xinu.at>

php check both, upload_max_filesize and post_max_size, when uploading a
file so we should do the same when figuring out the maximum size.

Signed-off-by: Florian Pritz <bluewind@xinu.at>

If the file wasn't uploaded successfully bail early.

Signed-off-by: Florian Pritz <bluewind@xinu.at>

Normally a browser window will be large enough that the bar is wider
than 100px so we can use decimals to display the real usage more
closely.

Signed-off-by: Florian Pritz <bluewind@xinu.at>