Add support to use merge request pipelines with GitLab Auto-DevOps templates

The annoying duplicate pipeline issues are starting to come to light as we develop our CI/CD pipelines further (see #48 (closed)). My vote has always been to use the suggested workflow rule to avoid duplicate pipelines, however, the GitLab Auto-DevOps jobs do not currently run in merge request pipelines meaning this workflow rule alone would break our pipelines as we depend on Auto-DevOps.

This issue is to implement a template in this repository which contains the workflow rule to switch between branch and merge request pipelines, and overrides to the rules sections for all Auto-DevOps jobs to allow them to run in merge request pipelines. Hopefully, this will resolve our current issues and allow us to standardise our pipelines on this workflow rule.