add MoocipController#isJSONRequest, refs #19
remove json_data from table mooc_blocks, refs #19

We use another table `mooc_fields` for that.
modify AbstractBlock#find to return the real type of Block instead, refs #19
put the cid into the Container as well
let BlockFactory#makeBlock return NULL if there is no such UI block class, refs #19
add a generic blocks controller as an endpoint to all blocks, refs #19