Restore "h" to hide or show files, and modernize file visibility toggling

Summary:
Ref T12616. This puts "h" back to collapse or expand the current file.

This removes some very complicated/messy code around following links in the table of contents and getting files auto-expanded. I suspect no one will miss this, but we can restore it if ayone notices.

Test Plan: Pressed "h" to collapse/expand a file. Also used the menu items.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12616

Differential Revision: https://secure.phabricator.com/D17940