[Aztec + GB] self-closing GB comments UI appearance

[mzorz]

copying from #678 (comment)

[from @iamthomasbishop ] I'm feeling like the best solution for now is to re-use the styling from an existing component - in this case, the more divider, and prepend "Block: " to the block type name. It would look like this (example shows of a shorter block type name and a long name):

We'll have different placeholder styles in Gutenberg, so I think for the time being (in Aztec), we can start here, and iterate if necessary.