Add gitlab issue/MR templates

As a minor point for housekeeping to ensure completeness of information, we could make use of GitLab templates for MRs (and issues). These are customizable to our needs. They prompt whoever is creating the issue or MR to fill out requested sections of information. I like also providing checklists of tasks that must be done in order to review/merge.

This addition is spurred by me realizing we missed some things in the CHANGELOG on recent MRs that should have been there before the MR was merged. A template that includes a checkbox for "updating change log" is a good reminder to both developers and reviewers.

I can make these templates easily if people are on board with adding these.