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. Testing Done: Ran unit tests.