use section instead of div for sidebar
~semantics matter~
adjust margin for header when sidebar is permanent

- tried playing with some JS to do this, but this is easier / more performant
- requires the sidebar to be the first element, which blows but is better than introducing JS for something CSS can/should handle ~imo~
use id instead of class for sidebar
add swipe to show sidebar

- uses basic touch detection to add no-touch class to desktops, will allow the to hover over sidebar at low res
- swipe from left of device will show sidebar, swipe from right of sidebar will hide it