move to gitlab

[fommil]

I'd like to move this repo over to gitlab, but I'll only do it if @VlachJosef, @lxol and @hvesalai agree. I've been trying it out recently and it has some features that make it better than github, e.g. integrated CI, free/libre license, more detailed settings (e.g. finer grain branch protection).

• [ ] copy over repo and issues (automated)
• [ ] max permissions to @VlachJosef, @lxol and @hvesalai
• [ ] set up new CI pipeline
• [ ] switch melpa to point to new repo
• [ ] update docs

[fommil]

nah, just swapping CI to use @chipsenkbeil's machine