Add Danger Python to GitLab CI

Add Danger Python to GitLab CI

Certain rules of the BIND development process are not codified anywhere
and/or are used inconsistently.  In an attempt to improve this
situation, add a GitLab CI job which uses Danger Python to add comments
to merge requests when certain expectations are not met.  Two categories
of feedback are used, only one of which - fail() - causes the GitLab CI
job to fail.  Exclude dangerfile.py from Python QA checks as the way the
contents of that file are evaluated triggers a lot of Flake8 and PyLint
warnings.

(cherry picked from commit 36bb45a8b661165430904eacaebed5ef8f08ba3d)