Add system checks for settings types

[Alexander Lazarević]

The discussion about this can be found on the forum at: ​https://forum.djangoproject.com/t/make-the-system-check-look-at-the-types-of-settings/27424

Use system checks to look for sane setting types at the start. Output would be something like this:

System check identified some issues:

WARNINGS:
?: (files.W001) The type of CACHE_MIDDLEWARE_SECONDS should be int (default 600). see https://docs.djangoproject.com/en/dev/ref/settings/#std-setting-CACHE_MIDDLEWARE_SECONDS

For some misconfigured settings there will still be an explicit ImproperlyConfigured exception raised (like The ALLOWED_HOSTS setting must be a list or a tuple) or an implicit exception like AttributeError: ‘int’ object has no attribute ‘find’ when for example set LANGUAGE_CODE to an int.

The implementation checks the type of the default value against the type of the user setting and issues a warning if they don’t match. This works for most settings, since they do have a default value other than None. For the ones that have None as the default value, a table is present, that contains the expected default values.

[Mariusz Felisiak]

Closing as "wontfix" as we haven't reached a consensus on the forum (at least for now).