Allow configuration for case-insensitive emails

Thanks for writing Djoser! I really like the library and API.

The only thing I’ve found frustrating recently is that emails are automatically treated as case-sensitive per discussion in #87. What I’ve had to do is override all of the default serializers with the following method:

    def validate_email(self, value):
        return value.lower()

I suspect that a lot of sites using Djoser store emails as all-lowercase. Would you consider a PR that would apply something like the following code? This would incorporate a default value of CASE_SENSITIVE_EMAIL = True but also allow me to easily change it globally for Djoser in terms of storing email, user login, password reset, etc. Happy to submit if it’s helpful. Let me know what you think!

    def validate_email(self, value):
        if settings.DJOSER['CASE_SENSITIVE_EMAIL'] is False:
            return value.lower()
        return value

[FendyPierre]

Is this a feature that’s being considered? I would like to make username case insensitive, but I don’t currently see any means of doing so. What are my options?

[YPCrumble]

@haxoza makes sense. As I think through it, I can’t think of a field other than email and username that would make sense for default sanitization. I also can’t think of a sanitization method other than case-insensitivity that makes sense to include in Djoser.

If you agree my suggestion would be to simply add two optional settings:

DJOSER_CASE_SENSITIVE_EMAIL = True
DJOSER_CASE_SENSITIVE_USERNAME = True

If you agree I’m happy to make the PR.