Add "unliking" posts on OnionrUI

If users want to undo their "like" if it was clicked on accident:

• if the block has not already been sent to a peer, the block hash storing the data saying that the block was liked should be deleted locally
• if the block has already been sent to a peer, an "onionr-post-like-revoke" block should be created stating the hash.

That brings me to a question for @beardog: Is there a way to determine whether or not a block that the local node created has been synced to a node yet? It would be useful for chosing between the two options above.