Add knowledge of branch freezes to the Release model

[Sebastian Wojciechowski]

Dear all,

I would like to start discussion about "How to express information about
release freeze to Bodhi".
This question is relelated to Bodhi issue
https://github.com/fedora-infra/bodhi/issues/1563

The initial plan was to just add some argument to cli "bodhi releases
create/edit", something like:

> > bodhi releases edit --name F29 --frozen true) 
but I would like to consider other options, if any, before I start working
on solution.

Sebastian Wojciechowski