Multi-content revisions

[lahwaacz]

See https://www.mediawiki.org/wiki/Requests_for_comment/Multi-Content_Revisions and https://phabricator.wikimedia.org/tag/multi-content-revisions/

• implement content handler stub (with contentmodel == wikitext assertion)
• update database schema: MW 1.31 introduced new tables: actor, comment, slots, slot_roles, content, content_models