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.

Testing Done:
- Ran unit tests.
- Edited diff settings and verified that it all still worked correctly.

Reviewed at https://reviews.reviewboard.org/r/14436/