Enhance development workflow for initial experiments usage

[jeddai]

The development workflow when developing and testing features tied into Nimbus has been made complicated by the initial_experiments.json file.

While developing and testing changes, developer channel defaults as set in the nimbus.fml.yml can be overridden by the application enrolling in first run experiments. This can be particularly problematic when, for example, a control branch for a first run experiment sets the value for a feature to the same value as the default, but a treatment branch sets a different value. The state of the application is effectively non-deterministic at that point, and can lead to confusion when inspecting code for errors.

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