Make GenericForeignKey subclass Field

[Adam Johnson]

When introduced in bca5327b21eb2e3ee18292cbe532d6d0071201d8, GenericForeignKey was created as its own class. Since then, it has grown to resemble a field, particularly in fb48eb05816b1ac87d58696cdfe48be18c901f16.

GenericForeignKey *not* being a Field leads to several issues:

1. It is added as a “field” and returned by Model._meta.get_fields(), a type confusion ​reflected in django-stubs.
2. It duplicates code, such as _check_field_name().
3. It misses methods like __repr__().

I think we should make it a subclass of Field.