frontend, cli, python: support multilib projects
    
    We create a new flask route for generating arch-specific repofiles.
    Using this, and using the new per-project knob Project.multilib,
    frontend can easily determine whether multilib repofile should be
    generated or not.
    
    For now, if the project is configured for multilib, we don't do any
    magic on backend.  We simply provide repofile with two (32bit and 64bit)
    repositories (the 32bit one contains `:ml` suffix in repoid).  And then
    - if user does 'dnf copr enable' against multilib project - both
    repositories are available on the system.
    
    This trivial approach can be pretty easily enhanced in future,
    especially if we wanted to support similar approach for per-package
    multilib detection (as e.g. pungi has), we'd only have to drop the
    second - multilib - variant of repo from repofile.
    
    The fact that we are adding a new routenow, and that we have the `:ml`
    suffix in repoid - we need to update the dnf-plugins-core package as
    well [1].  But for compat with previous versions of dnf-plugins-core we
    keep the old routes as well.
    
    [1] https://github.com/rpm-software-management/dnf-plugins-core/pull/350
    
    Fixes: #1
    Merges: #938