Allow to write server announcements

[STK-FabianF]

Description

It would be helpful for players with hammer (or /power2?) to be able to write server announcements, i.e. some kind of message the server sends, rather than the player itself.

[STK-FabianF]

It would also be helpful if the server could automatically send custom messages after a certain time, e.g. a certain message after 1 minute of playing time, another one after 10 minutes, and so on.