Pseudo-Randomize "every hour" and "every day"

[philister]

This replaces #596 , but the idea is the same: "If you use the Symbol-Shortcut (:hour or :day at the moment) and leave the at-argument blank, the exact time of execution will be randomized. This can reduce the load, when multiple jobs run hourly, for example."

Now its using a pseudo-random schedule-time, which means, that the same command will result in the same time. (So that times will not change between deploys for example)