Fixed #23583 -- More selectively ignored static/media roots

Fixed a regression introduced by 28efafa24c.
Thanks Michal Čihař for the report and initial patch, and
Collin Anderson and Tim Graham for the reviews.
Fixed #23663 -- Initialize output streams for BaseCommand in __init__().

This helps with testability of management commands.

Thanks to trac username daveoncode for the report and to
Tim Graham and Claude Paroz for the reviews.
Made testing of stdout and stderr more consistent.

Refs #23663.