Formerly, the settings were:
* `MFE_CONFIG` for common config.
* `MFE_CONFIG_<APP_ID>` for app-specific overrides,
with each app getting its own Django setting.
This commit changes it to:
* `MFE_CONFIG` for common config (unchanged)
* `MFE_CONFIG_OVERRIDES` for app-specific overrides,
where each app gets a top-level key in the dictionary.
Why the change?
* We want common.py to have a complete list of overridable settings, as
it helps operators reason about configuration and allows us to generate
config documentation using toggle annotations. Dynamically generating
setting names based on arbitrary APP_IDs makes this impossible.
* getattr(...) generally makes code more complicated bug prone. Tools
like pylint and mypy cannot effectively analyze any code that uses
dynamic attribute access.
