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.

If CHANGELOG exists, only the HEAD section will be updated.

I've also tightened up the way it lists commits, and fetched MR
data more efficiently from gitlab.

Generate a changelog

See merge request openflexure/openflexure-microscope!313