Add a Merge Request Template

Goals

This merge request introduces a merge request template for new MRs to use.

Non-goals

this merge request does not touch on issue templates

Implementation

This merge request template is based on @viktor-rosenfeld 's description of the multiple file input into moodle-tools. An explanation on how gitlab templates work can be found here https://docs.gitlab.com/ee/user/project/description_templates.html