Chosen tags

Changed over the Tag field in the editor to be a multiple select Chosen field. The field loads all the tags used within Storytime and allows them to be searched through. 

Added some JS magic to allow for the addition of new tags within that field - type in a tag name, if that tag is not found in the list just press enter and it will be added to the list of options. Create/update the post and any new tags will be created.
Tried adding it to the permitted_attributes in the post policy, but ran into issues with the association of the tag_list key with an array. 

```ruby
attrs = [:title, :draft_content, :draft_version_id, :excerpt, :featured_media_id, :published_at_date, :published_at_time, :tag_list => []]
attrs << :published if publish?
attrs
```

I don't think that Ruby liked the splatting of an array containing the assignment of a key to an array - confusing.