* dev/releases/utils.py: move GitHub specific code to new file so that
  one can use the utils code without having the "github" python package installed.

* Rename release_notes.readme.md -> README.release_notes.md

* use `from utils import error, notice`

* New script for generating release notes

* Tweak dev/releases/create_stable_branch.py

The 'tee' and subshell combo ate the exit code. Also get rid of the
'dev/log' subdirectory.

... so that one can use the utils code without having the "github"
python package installed.