In most cases, relying on rules instead of only/except clauses offers
more possibilites.

This commit changes the CI pipeline configuration as follows:

- Pipelines are executed with the Ruby rspec jobs for all branches,
except if an associated MR exists.

- In this case, create a so called "detached pipeline" with all, the
Ruby rspec and MR-specific, jobs.

- Pipelines aren't executed for tags at all, as the relevant commits are
tested in any case.

This prevents duplicated pipelines and therefore wasted resources.

Besides this, the CHANGELOG job result gets more reliable: up until now,
the result of it was timing-sensitive, as it queried the GitLab API to
search for an associated MR, to get the target branch to compare
against. This would fail in case a branch was pushed, the job started
but a MR only created some time afterwards, as there would be no target
branch, yet.

Now, as the job is run within the "detached pipeline" of a MR, and
relies on the MR-specific target branch exposed via a variable, this job
isn't timing-sensitive anymore.

To ensure these changes do work as expected, they were extensively
tested in a forked repo.

Closes #469