Expose game paused to Lua; disable bulletin board when paused

Work towards closing #2732.

One remaining task is to give some visual cue to the user that the
bulletin board is no longer clickable.

Another could be to wrap the 'disabled on paused' semantic into
something more reusable for other UI. I think this is out of scope for
While technically correct in a certain sense (as in acquire/release), I don't think ```ReleaseAdvert``` is a good name as it can easily be confused with ```RemoveAdvert```. Maybe call the two functions something like ```LockAdvert```/```UnlockAdvert```.