helm: Support pre-existing external services instead of subcharts.

[alexmv]

Add the ability to use external (pre-existing) PostgreSQL, RabbitMQ, Memcached, and Redis servers instead of always deploying Bitnami subcharts. Each service gets a <service>.enabled condition in Chart.yaml and a corresponding external<Service> values section for configuring the external connection details.

When <service>.enabled is set to false, the subchart is not deployed and the Zulip container is configured to connect to the external host specified in the external<Service> values.

The default behavior is unchanged — all subcharts deploy by default with enabled: true.

Fixes: #360.

Fixes:

How did you test this PR?

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[mikkeschiren]

I do like this suggestion, and also the way we want to deploy Zulip (and any other application). I haven't tested the merge request itself - but the solution - thumbs up.