For production code, the `ExtensionManager` instance is typically created as part of application init, because it needs to contribute to URLs. This was causing database access to occur before apps were fully ready, spewing warnings. This change adds a `delay_init` parameter to the `ExtensionManager` constructor. When this is set to `True`, we hold off on creating the `GenerationSynchronizer` until later. For unit tests, we pass this in as `False`. For real code, we pass this in as `True` and then the `load()` method will end up finishing the initialization. Testing Done: - Ran unit tests. - Ran the Review Board devserver (with some matching changes to that codebase) and saw no more errors about early database access.