AJAX-ify event preference selection

* Update event preference link helpers to use `remote: true`
* Update event preference link helpers with `id` attributess for JS
* Add helpers for event preference text (with `span` tags for JS)
* Update `links` partial to use new helpers
* Update `EventPreferencesController` to render a `.js.erb` view
* Create `event_preferences/create.js.erb` to swap links and text

https://trello.com/c/uk38y9x4