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+
Yes, that's true, but `.history_item.expandable`s have their own `.expandable_header`s, so I think it might be unclear whether `.history-expandable_header` applies to those, as well.

Perhaps this is an indicator that `.history-expandable` is not descriptive enough, though. If it was `.history-section-expandable`, then `.history-section-expandable_header` would be unambiguous.

Regardless, unless we think that these styles are useful in another context or that there's any realistic possibility of introducing some kind of wrapping element between the two things, I don't see the point in creating yet another class. Direct descendants are to be used with caution, but I think it's warranted here.