Use createBlock helper in place of manual block creation

[aduth]

Closes #688

This pull request seeks to use the wp.blocks.createBlock helper consistently for creating blocks, particularly in block transforms.

Testing instructions:

Verify that there are no regressions in changing between heading, text, and quote block types.

[youknowriad]

This means all the blocks returned already have uids. We may want to update switchToBlockType helper to avoid recreating new uids.

[aduth]

This means all the blocks returned already have uids. We may want to update switchToBlockType helper to avoid recreating new uids.

Isn't that what's already happening here?

https://github.com/WordPress/gutenberg/blob/90cc25b/blocks/api/factory.js#L88-L90

[youknowriad]

Isn't that what's already happening here?

@aduth no I'd update this to :

index === firstSwitchedBlock ? block.uid : result.uid

[aduth]

Thanks for the pointer @youknowriad . I've updated the factory method and updated the tests to assume blocks to be returned via createBlock.

[youknowriad]

LGTM 👍