Add settings to restrict end crystals and other blocks

Realms have a "TNT explodes" setting and a "Respawn blocks explode" setting, but nothing covers other ways to cause explosions or grief. End crystals still explode with both settings off, and there is no way for a Realm admin to stop them. 

Suggestion: add a block blocklist to Realm settings, so admins can restrict specific blocks and items instead of Mojang adding a new toggle every time one causes problems. 

Options could include:

• Disable placing or using the block/item entirely 
• Disable the block/item's explosion or damage behaviour
• Limit use to certain permission levels, such as operators only

Why this is needed:
Realms only have individual toggles for specific behaviours, and they leave gaps. Anyone who wants to grief can use an end crystal, and Realm owners can't stop it without removing players or restoring backups. Each gap needs its own new setting, which is slow and never covers everything. 

A blocklist would give owners one place to control all of this, including blocks that don't have a dedicated setting yet. It would suit any kind of Realm, from a small survival world with friends to a larger community, and it would make Realms safer for younger players without relying on trust or constant backups. 

A simple UI would be enough: a searchable list of blocks with a few toggles per block.