Preliminary support for seperate reporting database

This commit implements a Django database router which routes each
Django application to a database whose name matches a key in the database
dict, falling back to the default database if no matching key is found.

This support is plumbed through to the config file via
database.reporting_* database connection config options.  These options
mirror ones available for the default database config. If
database.reporting_engine is not specified in the config, then the
configuration falls back to the traditional single-database way of doing
things with the database router becoming a no-op.