frontend, doc: more scripted fedora branching
    
    This change adds `manage.py branch_fedora` command - that is a more
    automated variant of manual rawhide_to_release command execution for
    each rawhide chroot.  The change also introduces convenience option
    `--deactivated` for `create_chroot` command, so we don't have to create
    the chroot and disable right after that (racy).
    
    Suggest in doc that we should newly use `branch_fedora` command, and
    explain why is important to run the command as soon as possible.
    
    Fixes: #955
    Merges: #973