Add test mode to rotation

[levb]

In this mode no messages are ever sent to users, but they are still logged, and can be seen by the channel admins.

Everything else should work exactly the same. This way, new rotations can be added, "tested" with forecast, fully set-up, and then announced to the users.

The switch from the "test" mode to "production" mode (and back, should it even be allowed?) should be announced to the users in the rotation.