Fix repeatable select2 field

* Repeatable select fields did not work (using select2)
* All the JS was just output for by the CMB_Select html method
* Hacky setInterval used to make sure it works

Solution
* Split into JS file and use the hooks provided by CMB.js for initializding
* Only output field options, which are stored in `cmb_select_fields` object (using a JS-safe ID)
* Same technique as used for WYSIWYG field
* Also hook in initialization script on cloned field action to fix repeatable fields.