Because polyculeconnect is a OIDC proxy, we need to know which auth
backend to use. This is provided using a query param or a form, so we
need to get it from our own middleware.
This commit adds the following elements:
- basic DB storage for the backends
- support for DB migrations and a first test migration (not definitive)
- middleware to get the backend from the request and put it in the
context
- test that the backend exists in the auth flow
