Rethink markdown tag

[lastmjs]

I added the markdown tag, only to realize it didn't have syntax highlighting nor does it work within a solution tag because of templating and such (the specific implementation of marked-element doesn't work well within a template). So, rethink whether we should even have a tag like this. If the implementation were a little better with marked-element, it would probably be fine, but now it's just legacy trash polluting everything.