Add a basic gitlab issue template

This makes the Bug template available in the new issues form. Ideally a majority
of users will select this template and then realize that they're in the wrong
repo.

Making this a default is a feature of the enterprise gitlab version only, see
https://docs.gitlab.com/ce/user/project/description_templates.html#overview

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
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