Request:
{
'page':

{ ... }

,
'blockId': '...',
'date': 0123456789
}

Improve previewBlockAction in BlockController.

• decode page structure,
• call schedule from PageService,
• find blocks using getBlockById from Page class,
• encode using blocksToArray from JSONConverter,

Response:

{
'block_id':

{ 'html': '...', 'json': '...' }

}

• validate page object in controller,
• extract validation logic into new PageValidator class.