Switch Admin Settings Lists to Select2.js

 - and use jquery.sortable to allow sorting
 - support for autocompletion
add site_setting choices

 - add site_setting choices lists to list entries
 - allows for handy autocompletion using the new select2.js UI
 - automatically merges plugin choices into existing list, allowing for easy extension
Merge pull request #2405 from ligthyear/select2-sitesetting

AdminUI: Use Select2.js for SiteSetting Lists – adds Autocomplete and DnD-Sorting