Adds 2 checkboxes at the top of userlist in the settings

To display / hide the two last columns in the user list in the settings. Fix #9367.

This is my first enhancement to owncloud. Please review the code and let me know about best practices. I'll be glad to make the code better.

Some remarks : 
 - I don't know if I used i18n correctly.
 - I created some code duplication for the handling of the two columns. Should we handle more columns, we should create a more generic function to reduce code duplication.

@PVince81 @jancborchardt