Our recent fixes to using the system's configured memcached settings
broke populate_db, because its hacky clear_database helper is called
with a hacked-up settings module.
We fix this by first moving this out-of-place code from models.py into
populate_db, and then saving the settings required to access memcached
so that we can use them in clear_database.
We also fix a mypy erorr in flush-memcached that matches the same
issue fixed in clear_database.
This fixes some issues that we've had where commands will fail is
confusing ways after the database is rebuilt because data from before
the database was dropped is still in the memcached cache.