fix HABTM select filters

Assuming Foo HABTM Bars, the below now works as you'd expect:
```ruby
ActiveAdmin.register Foo do
  filter :bars
end
```
use the form buffer!
better filter defaults

+ `filter :attr, as: :select` now plucks DB values by default
+ properly add default polymorphic filters
+ move common Formtastic overrides into a single module
Merge pull request #1928 from Daxter/1093-dropdown-for-habtm

[1093] HABTM Select Filters