frontend, backend: fixed project forking
    
    This should fix broken project forking. In order to fix it, I had to
    change the format of datas sending from frontend to backend.
    Instead of lists containing names of old and new directories I am
    sending dictionaries, where keys are old directories and values are
    the names of new forked directories.
    
    There is also unit test for this.
    
    Fixes: #120, #652
    Merges: #943