Superflat customization screen ignores item rarity

Closely relates to MC-130145, MC-132199, MC-132201, MC-139386 and MC-227915.

Blocks that have an assigned rarity color should display this at all times. However, this is not the case for superflat customization, where they are all shown as white.

Can be easily reproduced via creating a world with this preset, and for future convenience, re-creating an existing world that has it:

minecraft:skeleton_skull,minecraft:wither_skeleton_skull,minecraft:zombie_head,minecraft:creeper_head,minecraft:dragon_head,minecraft:player_head,minecraft:beacon,minecraft:conduit,minecraft:dragon_egg,minecraft:spawner,minecraft:barrier,minecraft:light,minecraft:structure_void,minecraft:command_block,minecraft:chain_command_block,minecraft:repeating_command_block,minecraft:structure_block,minecraft:jigsaw;minecraft:mountain_edge

All of these blocks have item forms which have rarity colors, easily verifiable through the inventory, however none are displayed in the customization screen.