ignore MAC meta files on import and also remove any such files from the ...

...course

STUD-1725
@waheedahmed @symbolist @adampalay 
this is out of sync w/ master now. If you really want to do it course by course, you want `query = query_for_course(course_key, category='asset')` I see no reason to do it course by course. I think you want something more like:
```
    if getattr(course_key, 'deprecated', False):
        prefix = '_id'
    else:
        prefix = 'content_son'
    query = SON([
        ('{}.tag'.format(prefix), XASSET_LOCATION_TAG),
        ('{}.category'.format(prefix), 'asset'),
        ('{}.name'.format(prefix), {'$regex': ASSET_IGNORE_REGEX}),
    ])
```