Better mobGriefing GameRule

Improvements to how the entity specific mobGriefing game rules are used:

• Entity names are no longer hard coded strings, the EntityList is now used to retrieve entity names. This makes adding rules for additional entities much simpler. However, as a result some of the rules have changed slightly now they are aligned with the correct entity names.
• The entity specific game rules are now an extension of the original mobGriefing game rule, rather than each entity requiring separate game rules the form "/gamerule mobGriefing <entity name> <value>" is now used. Auto-completion is available for the new game rule form.
• A new value of "inherit" is now supported for entity specific mobGriefing game rules, this value will cause the entity to directly use the value of the original mobGriefing game rule. This is the default value and entities should use this when the behavior should adapt to changes in the global rule, rather than staying constant true/false values.
• A migrator has been included which will migrator the old format of the game rules to the new format. Any entities with a mobGriefing value matching the original mobGriefing rule will be migrated with the value "inherit".