re-define recurring event to offering relationship

[stopfstedt]

Currently, the defined relationship is a one-to-many from recurring event to offering.
I suppose the original idea was that recurring events are reusable recurrence patterns that can be applied to various offerings.

However, this is not how it works.
Each offering may define its own recurrence pattern.
This should be reflected in the schema as a one-to-one relationship.

Fix this in the database schema and code as applicable.

[stopfstedt]

superseded by #932.