Fix issues with Differential file toggling

Summary:
See D2714#15.

1. Give anchors an id equal to their name so JX.$ can find them.
2. Listen for click s on <a> s instead of hashchange s to make toggling a bit more correct and consistent.
3. Add a placeholder menu item when the file is unloaded. A previous patch by @jungejason had left the item blank in that case.
4. In fixing (1) I found another exception when clicking on ToC links. Since those links are differential-load, they try to load the file before jumping to it. However, loading destroys the node it's looking for, so if you jump to an already-loaded file JX.$ complains at you.

Test Plan: Make a giant diff. Click on links and try toggling files.

Reviewers: epriestley, vrana

Reviewed By: vrana

CC: vrana, aran, Korvin

Maniphest Tasks: T370

Differential Revision: https://secure.phabricator.com/D3185