Set I18n.enforce_available_locales = true
Removes `render_essence_editor_by_type` helper.
Refactors admin/elements_helper_spec
Removes unused helper `contents_form_field_ids_string`
Set `I18n.enforce_available_locales = false` in test env.
Removes `page_selector` helper method.

This is a ugly, hard to test, proxy helper, with no value at all.
We have `pages_for_select` helper that does everything we want.
Converts `admin/essences_helper` hash syntax.
Adds missing specs to `admin/essences_helper`