Add Danger Python to GitLab CI (v9.11)

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 36bb45a8)

Closes #1164 (closed)