Related: LB-766

There are a number of different ways that we connect to external services on app startup/serving a request (postgres, timescale, redis, rabbitmq).

Most of these just use a global variable to store a connection handle, but these connections are made in different ways for each service. 

We should use flask extensions or similar to handle connections for all external services. Hopefully this should give us a clearer idea of which views/modules need to use specific external services, and improve testability.