Issue #2270815 by vijaycs85, Gábor Hojtsy, nvinayvinay: Make schema testing code available as generic API.

Issue #1920862 by martin107, Mixologic, cilefen, larowlan, xan.ps, tim.plunkett | sun: Rename custom_block.module to block_content.module.

Issue #2274175 by tim.plunkett, vijaycs85: Block plugin schema should be defined separately from the entity.