[frontend] separate manage.py commands to their own files
    
    IMHO the content of manage.py file is getting out of control. I am
    moving all the *Command implementations to their own files to gain
    better readability and orientation in the code, but also possibility
    to implement helper _stuff_ needed for the command.
    
    Some of the commands are quite long and contain some logic.
    Such logic needs to be implemented on the class level, which can
    be annoying (e.g. some additional helper class needs to be added as
    a nested class). Ofc it can be put on the module level and mixed among
    command classes, but this solution isn't very good. Another solution
    would be putting such code into `coprs.logic` or `coprs.helpers`, but
    it doesn't make much sense, if its only purpose is to work within the
    command.
    
    Therefore, I am separating the commands into their own files to get
    rid of any limitations of their implementation.