Fixed #16161 -- Added `--clear` option to `collectstatic` management command...

Fixed #16161 -- Added `--clear` option to `collectstatic` management command to be able to explicitly clear the files stored in the destination storage before collecting.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16509 bcc190cf-cafb-0310-a4f2-bffc1f526a37