Sanitizes Markdown that goes back and forth between the server and
client side, to strip out data: links, so that they cannot be abused.
There is no present vulnerability to this issue–modern browsers disallow
data links in the first place, and we already filter this out in both
client-side code as well as the HTML generated in the REST API (it's run
through bleach). But we're adding this anyway, to further reduce the
odds that some client-side mistake could cause a vulnerability. This is
part of TNL-8589.