Let's add a model for NimbusBranch that foreignkeys onto NimbusExperiment and has the following fields:

• name: charfield
• slug: charfield
• description: textfield (optional)
• ratio: positiveinteger (default 1)
• value: textfield
• enabled: booleanfield

and add a test factory.

┆Issue is synchronized with this Jira Task
┆Issue Number: EXP-314