This sets up GitLab CI to automatically push CE master changes into EE
master, or revert them if the changes cause merge conflicts. The CI
configuration contains a single job to do this: `merge:master`. This job
is executed for every push to master, and periodically using a CI
schedule.

The periodic job is necessary because incremental jobs may not be able
to revert commits if newly added commits depend on these commits. By
re-running the job periodically (including all changes since a large
enough time frame), we can ensure that such commits are also reverted
(if they still conflict at that time).

The job runs in its own "merge" stage, _after_ the build and prepare
stages, but _before_ running the tests. This ensures that randomly
failing tests won't prevent code from being merged into EE. Running the
stage after the "prepare" stage reduces the chances of the job reverting
CE changes just because it ran before a corresponding EE MR was merged
into EE master.