Adopt the Lanyon disappearing sidebar for smaller screens

[PeterWhittaker]

Refer to changes in working files not yet staged.

Also, note that hyde has sidebar-about, Lanyon has sidebar-item. There are other little differences as well. Sigh.

Just dropping in lanyon.css (after removing some bits) and adding the button and button label to defaults causes lanyon to work as is. Things I prefer about Lanyon:

1. Smaller sidebar font

Things I prefer about Hyde:

1. No horizontal lines between sidebar items
2. Better centered container (is this not in CSS?)

Need the pure Lanyon way of sliding over the content, not pushing it out of the way - but on a smaller device, it might make more sense to push it out the way, since that would be down.

On the computer, older/newer are horizontally arranged, on the mobile, they are vertically stacked. Is this a font size thing?

[PeterWhittaker]

Commit 100a679 was the first experiment with this, it worked pretty well. Inconsistencies (why was hamburger slightly different for different widths, e.g.?) and aesthetic bugs were fixed in mostly in commit 8e15d0d and commit 395c089.