Copy/pasteable rich text blocks

[benmerckx]

When we copy/paste data from the rich text field we'll currently get the id of the block, but not the data. It can be pasted but will result in an error because it can't find that id. Also duplicating a block is a problem since they'll point to the same id. We need to either post process the code that is copied (not sure if that is even an option) or handle this in the parseHTML method of the block plugin.