Generate a changelog

At the moment, this simply lists the merge requests in each release, with descriptions and links.

More description is possible, e.g. categorising the MRs as bug fixes or new features, but that is left for the future.

It seems slightly silly to commit a file that can be automatically generated - but I think there is value in preserving this in case we migrate from GitLab, and there is also value in putting it into a text file that leaves room for extra annotations etc.

I think I should probably include some logic that will generate the file incrementally (e.g. will look for the latest tag in the changelog, and only add changes since that tag). Once that's done, I'm otherwise happy this works.

• Don't overwrite the existing changelog
• Only generate the changelog relative to the last release mentioned