Model and API changes for Firefox Labs Integration

[data-sync-user]

The NimbusExperiment model needs to be updated to add fields for the schema changes in phase 1. These fields need to be added to the v6 API and the v8 API.

No API or UI changes need to be made for this. The relevant fields will have to be made available in the Django admin panel for editing. (The Nimbus front-end is currently undergoing a rewrite and any new code for the old UI would be thrown out soon, resulting in wasted effort. Instead, we can live with a Django admin panel solution until the new UI is released since we expect low throughput of Firefox Labs opt-ins.

┆Issue is synchronized with this Jira Task