Fix issue #7152

Fix Issue #6683

Whent trying to upload/rename/create files in a folder that was removed
or rename, the correct error message is now shown.

In the case of upload of multiple files, the upload is cancelled.

This situation can happen if the target folder was renamed or removed
from another browser window or client.

OC_Util::setupFS($user) will create a data dir for the given string - no matter if the user really exists - OCP\JSON::checkUserExists($owner); introduces a ready to use check which will bail out with an JSON error

Fixes #5534

Fixes #5370

Fixes #5328

when _ is left in namespace and files are named after their classes the autoloader will also find classes in the lib folder of an app
its magic!

on reshares we now recursively move to the root of all reshares - therefore some code has been refactured and added as a new public function

When this is the case only the configured max upload size is taking into account for uploading

DRYing the code by using \OCA\files\lib\Helper::buildFileStorageStatistics()
now returning used space percent on each ajax call