Fix lazy translation of ListField errors

[rpkilby]

Spun this off of #6644.

• Fixes lazy translations for ListField.
• Adds a regression test to ensure that fields with lazily translated validation errors can be imported and instantiated during class creation. This test fails without the corresponding fix to ListField.
• Clarifies purposes of tests.importable

Test failure snippet:

INTERNALERROR>   File "/Users/bagel/Documents/projects/django-rest-framework/tests/importable/__init__.py", line 10, in <module>
INTERNALERROR>     class ExampleSerializer(serializers.Serializer):
INTERNALERROR>   File "/Users/bagel/Documents/projects/django-rest-framework/tests/importable/__init__.py", line 16, in ExampleSerializer
INTERNALERROR>     listfield = serializers.ListField(min_length=1, max_length=2)
INTERNALERROR>   File "/Users/bagel/Documents/projects/django-rest-framework/rest_framework/fields.py", line 1613, in __init__
INTERNALERROR>     message = self.error_messages['max_length'].format(max_length=self.max_length)
INTERNALERROR>   File "/Users/bagel/Documents/projects/django-rest-framework/.tox/venvs/py37-django22/lib/python3.7/site-packages/django/utils/functional.py", line 151, in __wrapper__
INTERNALERROR>     res = func(*self.__args, **self.__kw)
INTERNALERROR>   File "/Users/bagel/Documents/projects/django-rest-framework/.tox/venvs/py37-django22/lib/python3.7/site-packages/django/utils/translation/__init__.py", line 79, in gettext
INTERNALERROR>     return _trans.gettext(message)
INTERNALERROR>   File "/Users/bagel/Documents/projects/django-rest-framework/.tox/venvs/py37-django22/lib/python3.7/site-packages/django/utils/translation/trans_real.py", line 285, in gettext
INTERNALERROR>     _default = _default or translation(settings.LANGUAGE_CODE)
INTERNALERROR>   File "/Users/bagel/Documents/projects/django-rest-framework/.tox/venvs/py37-django22/lib/python3.7/site-packages/django/utils/translation/trans_real.py", line 198, in translation
INTERNALERROR>     _translations[language] = DjangoTranslation(language)
INTERNALERROR>   File "/Users/bagel/Documents/projects/django-rest-framework/.tox/venvs/py37-django22/lib/python3.7/site-packages/django/utils/translation/trans_real.py", line 97, in __init__
INTERNALERROR>     self._add_installed_apps_translations()
INTERNALERROR>   File "/Users/bagel/Documents/projects/django-rest-framework/.tox/venvs/py37-django22/lib/python3.7/site-packages/django/utils/translation/trans_real.py", line 139, in _add_installed_apps_translations
INTERNALERROR>     "The translation infrastructure cannot be initialized before the "
INTERNALERROR> django.core.exceptions.AppRegistryNotReady: The translation infrastructure cannot be initialized before the apps registry is ready. Check that you don't make non-lazy gettext calls at import time.

[rpkilby]

Going ahead and merging - I don't think any of this is controversial.