Adopt a merge request template

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

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.