First shot at an automatic ransack implementation

[jvdp]

I forgot what else I wanted to add before I went on vacation so I'll just post this now.

This PR adds the ransack gem and a filter_fields directive to Brightcontent controllers. By default, this gives you a dropdown with all the (unique) values in the database but you can also override this with a partial named _filter_field_* as is customary.

[jvdp]

I moved some things around to make naming more consistent:

• _index_top.html is replaced by _index_header.html and contains the resource name and "new resource" button (you can add eg. CSV buttons here without replacing _index.html.) For backwards compatibility, _list_header.html loads _index_top.html
• list filters are added below the header

[egbert]

Did something like this recently, not sure if I like it better. Although this implementation is a bit cleaner, the interface is a bit less clear. I think here we'd want a nice DSL

class << self
  attr_accessor :list_fields, :form_fields, :default_fields
end

@list_fields = [:title, :body]

[stefanroex]

Could you explain what the advantages or disadvantages are? I personally prefer the class method approach because it gives much better DSL then defining class instance methods. Imagine defining ActiveRecord relations like that...

[egbert]

As I said, the DSL is a lot nicer as it is. I think the other implementation is a tad cleaner though, without class_eval. But I think the nicer DSL is what counts here.