Randomize planet phase angle

Currently all procedurally generated star systems have planets with phase angles set to 0, meaning that they start at the orbit's periapsis. This randomizes it.

Noted in #3133

Break saved games
This means, you can't set the orbital phase explicitly to zero from the custom body, right?

It would be better to use a flag, if the orbital phase was explicitly set in the custom body and only randomize it if not.