add select2 to app specific thirdparty
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
fix deletion of shares
Merge pull request #10558 from owncloud/ajaxify_user_list_for_files_external_stable7

Ajaxify user list for files external stable7