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)
What about:

```ruby
report_params = params.fetch("aaib_report", {})

report_params.merge(
  "aircraft_category" => filter_blank_multi_select(
    report_params["aircraft_category"]
  )
)
```

It's still a bit meh, but I can't think of a way to do a nice select and map thing.