In Differential standalone views, disable some keyboard shortcuts which don't work

Summary:
Ref T13164. See PHI693. In Differential, you can View Options → View Standalone to get a standalone view of a single changeset. You can also arrive here via the big changeset list for revisions affecting a huge number of files.

We currently suggest that all the keyboard shortcuts work, but some do not. In particular, the "Next File" and "Previous File" keyboard shortcuts (and some similar shortcuts) do not work. In the main view, the next/previous files are on the same page. In the standalone view, we'd need to actually change the URI.

Ideally, we should do this (and, e.g., put prev/next links on the page). As a first step toward that, hide the nonfunctional shortcuts to stop users from being misled.

Test Plan:

• Viewed a revision in normal and standalone views.
• No changes in normal view, and all keys still work ("N", "P", etc).
• In standalone view, "?" no longer shows nonfunctional key commands.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13164

Differential Revision: https://secure.phabricator.com/D19571