Extends the discount level on renewal

[JarrydLong]

Extends the discount code expiration date and uses when the main level is renewed.

Resolves #100 Resolves #111

Steps to test:

1. Sign up for the main level and ensure that a discount code is created
2. Sign up for the same level. The discount code expiration date and uses should be pushed forward.
3. This will only happen when the main level is a recurring level. Once off levels don't have any effect on this.