Make banners waterloggable

Banners are amazing for customization of builds and it feels like the fact that they can't be waterlogged is a huge missed opportunity. I want to add banners to my underwater builds!

Also, since banners take up only one tile of space but are visually two tiles high, placing them underwater currently causes this really annoying effect where half the banner is in the water and the other half is in its own air bubble. So banners probably have the ugliest non-waterlogged state of any block, and waterlogging them could easily fix that.