mobile: tapping outside the sidebar should close it

[jsha]

Steps to reproduce:

1. On mobile, tap the upper-left hamburger icon to open the sidebar menu.
2. Tap somewhere on the right side of the screen (e.g. the search bar).

Expected result:

Sidebar closes.

Actual result:

Sidebar doesn't close.

[jsha]

Also, relatedly: Clicking a link in the sidebar that goes to the same doc (for instance, methods on a struct page) should close the sidebar.