replace chosen with select2 to provide ajaxified user and group selection for files_external, fixes #7499

remove minified select2 js

show avatars for users, simpler results

remove unneeded users and groups from settings template

fix css, escape user and group names

As constants not defined within a class cannot be automatically found by the
autoloader moving those constants into a class makes them accessible to
code which uses them.

Signed-off-by: Stephan Peijnik <speijnik@anexia-it.com>

Since the server doesn't support sorting, use client side sort directly
instead of calling reload() first.

Whenever a folder has a "data-share-owner" attribute, the icon is now
properly updated to a shared folder icon.