get dropdowns working for HABTM (WIP)

The plus to these changes is it's no longer necessary to specify `id`
in your filter. Assuming Foo HABTM Bars, the below now works as you'd
expect:
```ruby
ActiveAdmin.register Foo do
  filter :bars
end
```

#### TODO
+ get multiselect working
+ force Formtastic to get rid of the mysterious hidden input that it's
adding