Add build mode only parental control

Would hide all features related to the online aspects of Blocksworld, i.e the publish world option, being able to play others worlds, etc.
If that's a bit much, perhaps create a parental controls section in the game settings with options to individually disable each of the aforementioned online features.

The issue names no files, tests, or entry points. First resolve whether this should be a single build mode or individual parental-control settings, then identify the existing settings and online features involved; done means those features can be disabled as specified without affecting offline building.

Written by the indexing model from the issue text.