Revision history for all objects (documents, news, etc.)

[carlbennett]

• [ ] Draft design for MySQL table
• [ ] Populate MySQL table with current revisions of all objects
  • [ ] Bonus: Use the event_log table to generate historical entries of all objects; need to decode json metadata in logs to retrieve previous content
• [ ] Implement changes to each respective object class (Document, NewsPost, etc.), updating templates as necessary to accommodate changes
• [ ] Update templates to include navigating previous historical entries