chart: add ability to hard-config API tokens at dev time

[krancour]

A while ago, we introduced behavior in the chart where certain secret values are generated randomly on initial install, but on subsequent helm upgrade operations, existing values are retrieved and reused. This has led to a more stable upgrade process.

This falls apart, however, in cases where there's no cluster to connect to to find existing secrets and you need the secrets to have stable, unchanging values. Namely this can happen when using Tilt (see #1910) at dev time since Tilt never does helm install or helm upgrade, but rather only does helm template and then applies the returned YAML.

i.e. We want this ability if we want to limit unnecessary refreshes of components at dev time just because some token got a new random value every time the chart is rendered.

Note that I did NOT surface this as an option in the values.yaml file because no one should ever hardcode these except to facilitate development.

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