Move FileSystemFinder ImproperlyConfigured errors to system checks

[Jerome Leclanche]

FileSystemFinder currently checks for the following on __init__:

• Whether STATICFILES_DIRS is a list or tuple, failing if it's not
• Whether STATICFILES_DIRS contains a directory equivalent to STATIC_ROOT

I'd like to suggest moving both of those checks to the Django system checks framework. As it is, they happen at runtime which delays when the error shows up.

[Ling-Xiao Yang]

​PR

[Tim Graham <timograham@…>]

In 0ec4dc9:

Fixed #27661 -- Moved FileSystemFinder's ImproperlyConfigured exceptions to system checks.

Thanks Simon Charette, Mariusz Felisiak, Tim Graham, and Adam Johnson
for review.

[Mariusz Felisiak <felisiak.mariusz@…>]

In b23e3a1c:

Refs #27661 -- Added Tags.staticfiles.

Follow up to 0ec4dc91e0e7befdd06aa0613b5d0fbe3c785ee7.