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)

Please test @ser72. Note that if you disable avatars in the config it is significantly faster.

@jancborchardt note that there will be a small visual glitch when the new element appear and before the multiselects are initialized. I tried to make them look as close as possible to the initialized one.

@blizzz are there any user backends that don't return a sorted list of users ? (I've disabled client side sorting for performance)