Allow configuring the database's identifier max length in the DATABASES setting

[Hugo Tácito]

Can we please define a MAX_NAME_LENGTH in the DATABASE settings instead of using a fixed number as we can see at the method django.db.backends.postgresql.operations.DatabaseOperations.max_name_length

Example of use:

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.postgresql_psycopg2',
        'NAME': 'test',
        'USER': 'postgres',
        'PASSWORD': '',
        'HOST': '127.0.0.1',
        'PORT': '5432',
        'MAX_NAME_LENGTH': '128'
    }
}

[Tim Graham]

As noted in the docstring of the max_name_length() method, "This implementation simply returns 63, but can easily be overridden by a custom database backend that inherits most of its behavior from this one."

We try to avoid the complexity of new settings or configuration points where other solutions already exist. We could possibly make it configurable in DATABASES['database']['OPTIONS'], but I don't know that this is such a common need that it's necessary. You could write to the DevelopersMailingList for a second opinion. Thanks!