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

...course

STUD-1725
@waheedahmed @symbolist @adampalay 
Also, I believe this should be defined in `LMS` instead: `lms/envs/common.py` since `CMS` imports `LMS`.
The reason is because MIT invokes the `import` management command using the `LMS` configuration setting.