Make index_name configurable

[rbreslow]

This PR adds index_name to the WhiteNoiseMiddleware.config_attrs object. This allows you to set the value of index_name within Django's settings.py when using a custom static root / url.

For example:

# Static files (CSS, JavaScript, Images)
# https://docs.djangoproject.com/en/2.2/howto/static-files/

STATIC_URL = '/assets/'
STATIC_ROOT = os.path.join(BASE_DIR, 'assets')

STATICFILES_STORAGE = 'spa.storage.SPAStaticFilesStorage'

WHITENOISE_INDEX_NAME = 'assets/index.html'

Connects #43

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Testing

TBD.