This change adds a new layers argument to SiteConfiguration.get().

This allows passing in a list of additional settings dictionaries, which

will be checked before the main settings dict and defaults dicts.
Callers can use this to add overrides for specific keys: for example,

one might want to have a global setting for the server, a setting for

tenant accounts, and even a user setting for individual users. This

would be accomplished by passing in

layers=[user_settings, tenant_settings].
This also does some additional cleanup and typing for surrounding code,

reducing the noise I saw in my editor.

Ran unit tests.