Both the initialize_db script and Alembic need to import all
DatabaseModel subclasses so that DatabaseModel.metadata has all the
models attached. Previously this wasn't being done properly, and the
intialize_db script wasn't creating the scraper_result table since it
hadn't been imported.

This commit creates a dedicated module to import all of those classes,
so that both those locations can simply import * from it instead of
needing to import all the models individually. This still isn't great
overall, but it should be less prone to mistakes at least.