Milestone 4: Protected branches

Add protected branches for all major MariaDB releases (10.3, 10.4 etc).

Definition: Protected branches mean that Buildbot testing must pass before code is allowed to be pushed into Github.

Stage 1 means disallowing pushes if tests do not pass. Drawback: It requires manual intervention on behalf of the developer, within the short time window of tests having passed and no further commits having been done by other developers.
Stage 2 means automatic pushing if tests do pass. This addresses the drawback of Stage 1. However, if the logic is not 100% correctly implemented, very difficult git correction processes may follow.

To make sense (i.e. for Protected branches to be protected), all developers need to "turn on protection" by using the New Buildbot and not just the old one, which does not have this feature.

Before New Buildbot can be mandated, Milestones 3 and 4 need to be implemented (and that is why they come before Milestone 5).