Periodically merge CE master into EE master

This sets up the merge train to periodically merge CE into EE, instead of doing so on every master commit. This allows us to work around the issues of GitLab CE builds using shallow cloning, as well as working around redundant pipelines being cancelled.

To do so, we move the "merge-train" script from CE into this repository and adjust it a bit to better fit this new setup.

Fixes https://gitlab.com/gitlab-org/merge-train/issues/1 Fixes https://gitlab.com/gitlab-org/merge-train/issues/6

TODO

• Mirror merge-train on ops.gitlab.net: https://ops.gitlab.net/gitlab-org/merge-train
• Configure ops.gitlab.net so that it allows CI schedules with an interval of 5 minutes: was already the case
• Move the configuration environment variables from CE into this ops repository
• Test using a fork of CE and EE
• Use the ours merge strategy for all conflicts (https://gitlab.com/gitlab-org/merge-train/issues/7#note_122427657 and https://gitlab.com/gitlab-org/merge-train/issues/7#note_122523362)
• Update CE developer documentation: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23689