Break up experiments/constants.py

[freshstrangemusic]

constants.py has become a dumping ground:

• Application configurations classes
• Channels
• Randomization Units
• Changelog events
• Firefox Labs metadata
• The Application text choices
• Application configuration instances
• The NimbusConstants class:
• a bunch of unrelated constants
• a bunch of unrelated TextChoices
• a hojillion error strings

This should be split up by use / function:

• Applications should go in a new "applications" Django app (experimenter/applications)
• Errors should go relative to where they are triggered (e.g., serializer errors should go in api/errors.py or similar)

etc.

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