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+
Sorry, I totally forgot about this discussion. After reconsidering, I think that having both `.history_item` and `.history-item` classes on one thing is a recipe for mountains of confusion down the road. Considering that `.history` by itself is not being applied to anything, I think it probably makes the most sense to rename all of the `.history_element`s to `.history-block`s, which will solve this problem.