Ability to show/hide last log in and storage location in user management

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.

I'm not sure that the checkboxes should be placed there. If they should be placed on another page, please tell me...

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