Adds specs for admin/pages_controller and refactors a little.

Page#unlock is now Page#unlock!
Page#fold is now Page#fold!
Adds spec for PagesHelper#picture_essence_caption and refactors it.
Adds specs for deprecated language_switches/language_switcher helpers.
Adds spec for PagesHelper#render_meta_data.
Adds spec for Admin::ElementsHelper#elements_for_select.
Removes duplicated method from Element model.
Removes unused AuthenticationHelpers module.
Removes unused helper method #render_classes
Element takes use of #display_name (same behavior as before)
Adds spec for page model and renamed #lock to #lock!
Refactors page model spec for #unlock!