Add selectize to has_many form fields

Created by: gracewashere

selectize.js:

• http://brianreavis.github.io/selectize.js/
• https://github.com/brianreavis/selectize.js

New dependency: selectize-rails

• https://github.com/manuelvanrijn/selectize-rails

Trello: https://trello.com/c/ETg2bieT