setup redis, as a service dependency for flask-limiter

In the context of T616, we need a backend where to store live usage statistics for various "users" of the Web API. After discussion with @olasd we picked Redis among the various backends supported by flask-limiter.

This task tracks the need of deploying and configuring Redis for our current rate-limiting goals.

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Migrated from T635 (view on Phabricator)