Replace Chosen with Select2

Select2 is much more configurable and supports remote data loading
which we may want at some point.
To that end:
* Added the Select2 files to a new plugins dir
* Removed Chosen plugin and stylesheet
* Replaced references to Chosen files with references to Select2 files
* Updated script.js to call select2 and pass in appropriate config values
* Added new placeholder key in config.js to give to select2