So we implement it ourselves by filtering out pre-selected items in the
processResults callback.

solution suggested on stack overflow - namely, as each selection tag is
rendered add a custom class if it's the principle user.

Relevant posts:
  https://stackoverflow.com/questions/16564425/how-to-implement-select2-with-lock-selection/48111387#48111387
  https://jsfiddle.net/o3s3y42e/4/

bugfix: "single select" control not initialising (introduced single_user on ucamlookup_users.html to initialise "single select")

The templates macros now support multiple calls: more than one lookup
input tag can be used in a template