Update the MementoEmbed logo to use <embed> rather than <em>

[shawnmjones]

This was suggested for Raintale by @machawk1 -- see oduwsdl/raintale#14. The <em> HTML tag was originally chosen because it is semantically equal to the word emphasis. The tag <embed>makes more sense with the Memento clock logo. If we make this change, we might as well update all DSA logos to incorporate the Memento clock logo. At the time I was creating logos, I did not have digital access to the Memento logo, so we would need to acquire that first.