sop_mass_branching: update to latest fedora-{release,repos} code, drop Taskotron
    
    The following changes are included:
    1. fedora-{release,repos} steps have been updated and changed to small snippets
       instead of a hude diff, which I believe increases readability (and also makes
       future updates easier)
    2. fedora-repos now needs to be updated in stable releases as well, to provide
       latest GPG keys and symlinks - instructions added
    3. Taskotron mentions are dropped, it is EOL
    4. Trailing whitespace got trimmed (because my editor does that automatically,
       and I can't argue with it)
    
    Signed-off-by: Kamil Páral <kparal@redhat.com>