Improve users list scrolling performance

- fixed JS error when avatar mode is disabled
- added spinner at the bottom of the table
- scroll detection now happens earlier
- single/multiselect init is deferred so that the new rows are first appended
  into the list (more responsive) and initialized afterwards
- disabled users sorting after add (assuming they are always sorted on
  the server side)
Added loading spinner to users list on scroll
Merge pull request #7397 from owncloud/stable6-scrollperformance

Users list scrolling performance / improvements