Manage Django choices & Enums with DjangoChoicesEnum

[phalt]

This PR aims to resolve some of the issues seen with choices and enums:

• Choice string converter should not force upper case #133
• Lowercased choices getting uppercased in Graphene response #280
• CharField choices mapped incorrectly to GraphQL #577
• Enum access #193
• Access to generated enum for Django Choices #240
• How to create a field to expose Model Fields' choices #471
• graphene-django expose form field choices as enum #505
• Override name for choices in a model field #225
• Reuse enum type of choices in serializer mutation #517

They all want different things, so there isn't going to be a solution that makes everyone happy. This PR proposes a solution that covers most of the complaints.

Major issue 1

Managing the difference between an enum and a choice list.

Django's choices accept a list of tuples. Graphene's enums don't support this. How can we find a happy medium?

Solution: Implement a helper class that manages the conversion for us.

from graphene_django import DjangoChoicesEnum

class MyChoices(DjangoChoicesEnum):
    FOO = 'foo'
    BAR = 'bar'

# Get a Django choices list
MyChoices.choices()

# Get a Graphene enum
MyChoices.enum()

Major issue 2

How do I see my enums?

We've documented a way of accessing enums through the API, in the documentation.

[coveralls]

Coverage decreased (-0.3%) to 93.654% when pulling beb422f on custom-choices into 223d0b1 on master.

[ProjectCheshire]

Hah! I had this same issue with neomodel choices

[zbyte64]

I wonder if it makes more sense to have DjangoChoicesEnum directly inherit from graphene's Enum , not sure if all the metamagic is in the way.

Edit: Another alternative implementation would be to have a utility function that generates django choices from graphene enums. This would feel less opinionated.

[zbyte64]

The user still needs to define the enum field at the DjangoObjectType for this to work, which I think is the proper way forward. We might want to document that use case.

[zbyte64]

Came across another project's attempt to mediate enums and choices, I like the usage pattern here: https://github.com/jazzband/django-model-utils/blob/master/docs/utilities.rst#choices