[#70952590]
[#70952590] Made offers STI and added an endpoint for offer types.

- Offers are now STI, and there are two offer type subclasses, "good"
  and "service". We may add others and / or make things more specific in
  the future, eg "lessons" could be good.
- The /offers/types endpoint now exists and returns a hash of offer
  types supported by the system. The hash is keyed with the class names
  and has human-readable values as value names. Great for populating
  select inputs.