Add History page with timeline

- Adds Sheer support for the History post type
- Implements single-column History layout
- Implements two-column History layout at 1200px+
Problem is that there are two levels of expandables in a `.history-expandable`, so that wouldn't clearly indicate that it only applies to the top level. This seems like an appropriate use of the direct descendant to me. Can you give an example where it might become problematic?