ci: Let Marge the merge bot trigger our CI

We have notice that a lot of CI activity is cause by user pushing to their
branch after having created an MR. To reduce our CI foot-print, the CI will
now only be automatically triggered when a reviewer assign the MR to the merge
bot. It will still be possible to run the CI manually but the result of that
CI won't be used by Marge.
40 jobs for master in 21 minutes and 20 seconds (queued for 9 seconds)