Remove customer_id relationship from CloudActivation

Problem

In #4800 (closed), CloudActivation should be associated with a BillingAccount, as well as a Customer (to be renamed User). However, the Customer association is not ideal and should be removed once the new BillingAccount structure is vetted and the feature flag is ready for promotion. As part of removing the feature flag, the CloudActivation association with Customer should be removed.

Proposal

Once the feature flag is ready to be removed, we should removed the customer_id foreign key from cloud_activations, but this needs to be done in several steps to avoid downtime.

1. MR 1
   • Remove not null constraint on customer_id column
2. MR 2
   • Remove code that uses or sets customer_id and ignore customer_id column
3. MR 3
   • Drop the customer_id column and the index associated
4. MR 4
   • Remove the ignore customer_id rule