Adopt a merge request template

The idea here is to encourage people who write commit messages that don't adequately explain relevant information to at least clean up the merge request description, which makes their merge requests more likely to be reviewed and makes life easier for reviewers. At merge-time, the MR can be suqash-merged to make the final commit message echo the MR description.

For people who already write good commit messages or send MRs of multiple curated commits, nothing really changes.

This has worked out very well for Elisa, which has used this template for a year. It's modeled on the template we used for Phabricator. Copy it here and give it a try for Plasma.

If it works out, I'd eventually like to do a sysadmin thing that automatically syncs this into every repo that opts in, so we don't have to copy it around. But at this point I think that's a bit premature until we get more widespread usage and acceptance.