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).