Instead of reimplementing show preview/diff/edit conflict/etc for editing collabkit, we should just subclass EditPage instead and get all that for free.

This will include hub creation. Consensus of the development team is that the creation logic should be handled in EditPage through ?action=edit&model=CollaborationHubContent, but should not be directly exposed in the UI. This is because making the EditPage UI accommodate multiple content models is a fundamental MediaWiki question we are not prepared to address at this time. So we will continue to use Special:CreateCollaborationHub for the creation form and have that feed into EditPage, which means we won't have to do as much wheel reinvention.