Refactors Page#publish spec to not use database.
Adds and refactors specs for ActsAsEssence mixin.
Removes unused method Element#paste_from_clipboard
Fixes issue with ElementsHelper#elements_for_select. Adds specs and refactors the code.

Issue was introduced by af0d042baefa0297a6af92cf289fded40d5cc9fa where Element#display_name was called on a Hash instead of an element instance.