Fixed #21581 -- Fixed a number of issues with collectstatic.

When STATIC_ROOT wasn't set, collectstatic --clear would delete
every files within the current directory and its descendants.

This patch makes the following changes:

Prevent collectstatic from running if STATIC_ROOT isn't set.

Added `STATIC_ROOT = os.path.join(BASE_DIR, 'static')` to the default
project template.

Fixed an issue that prevented collectstatic from displaying the
destination directory.

Changed the warning header to notify when the command is run
in dry-run mode.