backend: ask for auto_createrepo once per project
    
    This pretty substantially minimizes the queries against Frontend.
    
    Motivation for this change though was that Action.run_createrepo()
    failed on createrepo() helper too late, after it already created the
    target directory (see 22eba237e57c76fc7021b4a7cb48af3c7e98b65f).  This
    led to leftover directory if the get_auto_createrepo_status() failed
    because we deleted project in the meantime (race).
    
    In the new code, if the uses_devel_repo() fails (only for already
    deleted project), we don't even create the directory and fail ASAP.
    This race was detected by Sanity test, after applying the previous
    commit;  so while I'm on it I'm moving the sanity test to a separate
    file.