require fallbackMapping when backgroundRate is <100?

[bhearsum]

Now that it exists, fallbackMapping is something we almost always use when running a throttled rollout. I can't think of any case where we'd want users to go no updates instead of the previously released version during these times. We should consider enforcing this in the backend, and rejecting any requests to change backgroundRate to <100 unless fallbackMapping is set, or is getting set as part of the change.

We'll need to ensure that these would actually work for all different types of updates (GMP, SystemAddons, Firefox) before deciding to move forward.

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1391013)