Add back pretty formatting of non-symbol attributes.

[kaspth]

3efb359, switched around a bunch of code, but also scoped pretty_format-ing to only
symbol attributes.

This broke formatting in our app. In Active Admin v1.0.0.pre2 code like:

show do
  attributes_table do
    row :mobile_subscription do
      subscription # ActiveRecord::Base object.
    end
  end
end

would auto link the returned model, but pre4 merely returns an inspect like output.

I think scoping pretty_format to only symbol attributes was an oversight as the
method clearly knows how to handle lots of different objects.

This refines format_attribute so we can still get nice status tags for boolean
fields, but get pretty formatted output for the rest of the values like pre2 and
earlier did.

[kaspth]

Okay, I was a little too quick to call it an oversight for my own good. Sorry about that! ❤️

I do think it would be nice to do someting about this, since the behavior in pre4 doesn't match the one in pre2 as shown above.