Make "DifferentialDiff->properties" a proper "attachable" property

Summary:
See PHI498. This should be initialized to "self::ATTACHABLE" like other attachable properties, but is currently initialized to "array()".

Initialize it the normal way and try to catch all code paths which may have accessed it without actually loading and attaching it.

Also, remove UI for the very old "excuse" property, which "arc" has not written for well more than a year.

Test Plan: Grepped for affected symbols, loaded various revision pages. Somewhat tricky to be 100% sure that every pathway is caught, but it should be obvious if I missed anything once someone hits the code path.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Differential Revision: https://secure.phabricator.com/D21710