Allow User to select multiple aircraft categories

This PR adds multiple selection for aircraft categories:
- adding select2 gem for doing nice multi selects
- removing partialised extra form fields that were left in

The `filter_blank_multi_select` is to fix a [known Rails issue](http://stackoverflow.com/questions/8929230/why-is-the-first-element-always-blank-in-my-rails-multi-select).

[Trello Ticket](https://trello.com/c/erWWpMKd/201-specialist-publisher-allow-editors-to-add-multiple-tags-make-it-publish-properly-to-rummager-and-content-api-3)
Actually, since we can run it on all of them, how about:

```ruby
def document_params
  filter_blank_multi_selects(
    params.fetch("aaib_report", {})
  )
end

def filter_blank_multi_selects(params)
  params.reduce({}) { |filtered_params, (key, value)|
    filtered_value = value.is_a?(Array) ? value.reject(&:blank?) : value
    filtered_params.merge(key => filtered_value)
  }
end
```