Add release issue templates

Add Release from develop and Release from maintenance GitLab issue description templates, following what cppTango does in .gitlab/issue_templates.

The two release task lists were so far only written out in how-to-contribute.md and had to be copy-pasted by hand into a new issue. They now live in .gitlab/issue_templates/, are picked from the Description drop-down when creating an issue, and the how-to only points at them, so the list exists in one place.