This adds a new setting, settings.EXTENSIONS_ENABLED_BY_DEFAULT, which

is a list of extension IDs that will be automatically enabled when first

registered in the database through ExtensionManager.load(). This

allows a project to automatically enable specific extensions, such as

first-party ones considered a part of the application.
Once an extension is later disabled, it will not be automatically

re-enabled unless the registration is removed from the database.

Extensions also cannot opt into being enabled by default.

Unit tests pass.
Tested this manually with an extension that was set to be auto-enabled.