SiteConfiguration.get() and .set() now support taking a list/tuple

of strings for a path when getting or setting a value in siteconfig

settings. This is used to manipulate settings within nested

dictionaries, making it easier and safer than pulling out a dictionary

and trying to replace it.
This is supported for stored settings and registered defaults. To make

this easier to manage, there's a new _UNSET value used for defaults,

and a couple helper functions to separate out stored setting retrieval

and default setting retrieval.
get() also now accepts an optional required argument, which will

raise a KeyError if a setting is not found in the configuration or

defaults. This can help callers distinguish actual None  values from

missing/bad settings.
The management commands have been updated to take advantage of this,

simplifying that code and ensuring we have only one code path where we

update nested settings.
Upcoming changes will build on some of this work.

Unit tests passed.
Made use of the new key path support in some upcoming changes in

Review Board.
Tested the set-siteconfig and get-siteconfig management commands

to make sure they worked as before.