Now also preventing to override "files" dir size with -1

Fixes #6526
[tx-robot] updated from transifex
Fix submodule 3rdparty - translation sync broke that
Merge pull request #6673 from owncloud/master-quota-insufficientstorageforfilesfix

Now also preventing to override "files" dir size with -1
Merge pull request #6677 from owncloud/fixing-3rdparty-submodule

Fix submodule 3rdparty - translation sync broke that
Merge branch 'master' into disable-mod_pagespeed