Add a MP gamerule to limit breaking/modifying signs made by other players

This is mainly for multiplayer servers. The game rule might be called "protectedSigns" and would affect both the breaking and modifying of signs (text color & glow).

Here are some examples for multiple possible settings:

"protectedSigns none" == all signs are breakable and moddable by anyone. (Minecraft default)

"protectedSigns peersOnly" == Signs placed by Operators cannot be broken/modded by anyone. Operators can break/modify signs placed by Members or Visitors, and Members can only break Visitors' signs.

"protectedSigns operatorsOnly" means signs placed by Operators cannot be broken/modded by anyone, but a Member's or Visitor's sign can be broken/modded by anyone.

"protectedSigns all" means all signs are protected, and only the player who placed a given sign can break or modify it.

Sign ownership is only recorded while "protectedSigns" is set to anything but "none". Signs created in an older world would be retconned to have anonymous Visitor ownership, although there could be some provision included during the conversion process for the Owner of an older world to take ownership of all pre-existing signs.