ClassificationFeaturings can be 'offsite'

Adds fields to ClassificationFeaturings and changes validation
so they can have either an edition or a title, summary and url
for an offsite page.
Offsite featurings visible on topic show page
Offsite featurings visible on topical events page
Don't create edition in offsite featuring factory
Tweak validations for clarity
Ensure required fields are labelled as such
Ensure offsite featurings are external links
advances
work in progress
presenters
offsite factory