Add a make command to create a Changelog entry

This idea was proposed in !2403 (comment 478924) .

In #3522 (closed), we changed how new entries are added to the Changelog. They should be added in a separate file in the changelog_unreleased directory rather than prepending them into the main Changelog file.

The developer must create an entry file manually based on already existing entry files, documentation, or the template file in the changelog_unreleased directory.

We can implement a make task to create the entry file automatically.

@andrei proposed:

What about a command, e.g. make add-changelog-entry, that creates the entry automatically with the branch name e.g. changelog_unreleased/3522-eliminate-merge-conflicts-on-changelog-md.md?

The content could be inline which would allow us to get rid of .template, and you could replace it with .gitkeep.

The command could also add the gitlab issue id.

The rule in core could also be responsible for adding a changelog entry in premium. It could take a flag or an environment variable for this purpose.