Draft: Add default Gitlab templates for Issues and MR's

See https://docs.gitlab.com/ee/user/project/description_templates.html#set-a-default-template-for-merge-requests-and-issues

When people create a new issue or MR, they will now get a default template. This can be used to give proper information to people about what we need/expect for issues/mr's.

You can test on Pleroma in my namespace (I set this branch as default branch) https://git.pleroma.social/ilja/pleroma

This is based partly on what we already had, partly on what we had for newroma, and partly new stuff I added. Pls let me know if there are things you think need changing so I can adapt it.

• Add some first draft
• Also peek at what was done for FE (seems pretty good) and update here pleroma-fe!1577 (merged)
• From @helene in chat: "you mix PR and MR a lot, and maybe specifying what those acronyms are at least one would be nice (writing "merge request" once instead of just MR, for example) :)"
• Also say something about changelog/docs
• Maybe check pleroma-meta!1 (merged) if there are other things that's checked during e.g. code review that can be added here