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+
There's a realistic possibility that we might refactor any given design or pattern, for example after doing user testing. I'd  like for our code to be flexible within reason and avoiding direct descendants in a lot of cases helps with that and is something that is easily done in this case.