R1:dbde4b9ff281

Fix issues with Differential file toggling

Summary:
See D2714#15.

# Give anchors an `id` equal to their `name` so JX.$ can find them.
# Listen for `click` s on `<a>` s instead of `hashchange` s to make
toggling a bit more correct and consistent.
# Add a placeholder menu item when the file is unloaded. A previous
patch by @jungejason had left the item blank in that case.
# In fixing (1) I found another exception when clicking on ToC links.
Since those links are `differential-load`, they try…