Refactor common mutation logic

[XanderVertegaal]

A lot of validation logic is now duplicated across our mutations, and it is easy to forget to add contributors. LettercraftMutation should take over some of the former and prevent us as developers from doing the latter.

[XanderVertegaal]

The current plan is:

LettercraftMutation should get new methods:

• add_contributors: raises NotImplemented if subsclassing mutations don't overwrite this function.
• validate_input: pass if not overwritten, validates the mutation input object.
• perform_mutation: contains all current logic of mutate_object.

The existing method mutate_object should, in sequence:

1. start an atomic transaction
2. call validate_input()
3. call perform_mutation()
4. call add_contributors()
5. validate the resulting mutated objects (with full_clean())
6. catch any errors
7. return the resulting/modified objects

Still unclear is how to get the manual updates (now done using excluded_fields) in this flow.