The class hierarchy for mutations was a mess. We had a few base classes

(BaseMutation, MonoBaseMutation, and MutateModelField), with a

hierarchy descending in that order. BaseMutation was the base class

for all mutations. MonoBaseMutation was for models. MutateModelField

was for fields on models. While the naming was inconsistent, that

hierarchy made sense.
There were a few problems, though. MutateModelField.mutate (the base

mutate function for models) really belonged in MonoBaseMutation, and

the lack of this made things pretty confusing. Worse, several of the

mutations that worked on models (and not fields) were inheriting from

MutateModelField.
This change renames the base classes to BaseMutation,

BaseModelMutation, and BaseModelFieldMutation, and fixes up the

order they're listed in the field and moving

BaseModelFieldMutation.mutate into BaseModelMutation. It also fixes

up the specific mutations so that they all inherit from the correct

base class.

Unit tests pass.