add length to text columns or newer mysql will produce columns too wide

setbackgroundjobsmode.php got lost somewhere - switched call to use the perferred OC.AppConfig.setValue() instead

https://github.com/owncloud/core/commit/2028c8116122f56d8087716e42162fdc
651819d3#commitcomment-1766755

OC_Util is better thank OCP\JSON because it'll redirect the user to the index.php instead of displaying an error

Fix URL to sources