tech-debt: merge Subscription + Contribution into 1

#### Description

- Review Current model Subscription / Contribution
- List out fields which are no longer relevant
- Remove those and migrate the rest into 1 table

@danlipert I might need some guidance on if this is worth doing and to what extent

This will be broken down into

- [ ] step1: remove unused fields + code which invokes them in Grant and Subscription
- [ ] step2: move remaining fields from Subscription into Contribution (custom migration)
- [ ] step3: update all referenced from Subscription to Contribution
- [ ] step4: purge Subscription