Implement urls domain, protocol and site name for frontend

[FraCata00]

If you're develop a backend and frontend in separate projects, cannot add ACTIVATION_URL in DJOSER settings like this

DJOSER = {
    "SEND_ACTIVATION_EMAIL": True,
    "SEND_CONFIRMATION_EMAIL": True,
    "USER_CREATE_PASSWORD_RETYPE": True,
    # specially this
    "ACTIVATION_URL": os.environ.get(
        "DJANGO_ACTIVATION_URL", "auth/users/activate/{uid}/{token}"
    ),
}

because implicitly check inside the backend this path from example localhost:8000, and maybe the frontend project is in another repo and another host

From documentation:

This PR #729 by @n-borges allows this functionality
Please check this out ❤️

[RodrigoSosa95]

For anyone trying to get this result take a look at this stackoverflow question

Adding DOMAIN and SITE_NAME solves it.

[FraCata00]

Great, I use it to solve it

[tomwojcik]

Sorry it took so long, merged. I will release it within a month with other minor improvements. Thanks!