Changelog automation

A CHANGELOG.md file is now present in the root of the project. A CI step was added which enforces a change to this file on every branch that has an open merge request. (This approach possibly has issues, for example if a MR is created after the commit pipeline runs; we could possibly run a detached merge request pipeline instead but it seems to override the results of commit pipeline entirely. We should discuss this in the meeting.)

The changelog file has a section at the top for an upcoming release. When a new release is made, this section is renamed to the version number of that release, and a new "unreleased" section is placed on top.

Each entry in the file must adhere to the following format:

- ([<Affected user type>])+ <Description> (<Issue/MR number>+)

This is not a proper definition, I know, but it's easier to read :)

For example:

- [Coordinator] Improve Java grouping algorithm functionality. (!1070, !1073)

Or:

- [Coordinator] [Company representative] Fix inability to remove all periods from a project. (#689)

Closes #724 (closed)