Refactor DocumentPresenter#fields to use generic logic applicable for different types of field displays

[cbeer]

This allows us to use common functionality for IndexPresenter + ShowPresenter, but also opens up a seam for view-specific field configuration (e.g. specific to atom/rss fields, gallery views, etc).

configure_blacklight do |config|
  config.view.gallery.document_fields_key :gallery_fields

  define_field_access :gallery_field, Blacklight::Configuration::IndexField

  config.add_gallery_index_field :field_1
  config.add_gallery_index_field :field_2
  config.add_gallery_index_field :field_3
end

[sandbergja]

@cbeer could you please rebase this? Thanks!