The DiffSettingsForm has a few things which are handled specially (in

order to serialize and deserialize certain settings where the UI doesn't

map trivially to the settings keys). These were written to read and

write directly from the siteconfig.
This change makes it so those use the base KeyValueForm

get_key_value() and set_key_value() methods instead. The

implementation of those ends up doing the same thing as the current

code, but will allow us to subclass this form in RBCommons and override

those methods to try reading and writing from the LocalSite instead.

Ran unit tests.
Edited diff settings and verified that it all still worked correctly.