Simplify the Configuration File

This PR removes the ``database.migrations`` and ``security.csp`` "options" which essentially would never change in any reasonable deployment of Warehouse and it simplifies the ``logging`` option into just specifying what log level you wish. This results in a smaller configuration file which is much easier to reason about and get "right".

This will require changes in deployments if there were custom log levels being implemented, but otherwise the old settings will just be silently ignored. /cc @ewdurbin