scripts: Add a gen_release_notes.py script

This script is responsible for generating an entire page in the
docs/relnotes/ directory. It includes a template for the page, and uses
mako to fill in the necessary bits. It is designed to be purely fire and
forget, calculating previous versions, shortlogs, bug fixes, and dates.

Acked-by: Eric Engestrom <eric.engestrom@intel.com>
Acked-by: Juan A. Suarez <jasuarez@igalia.com>
(cherry picked from commit 86079447)