Add syntax highlighting to "View full code" modal

[rabidbit]

Currently, if you click "View full code" link in user-submitted code speech balloon, the modal/pop-up containing the submitted code does not have syntax highlighting. User feedback prefers syntax highlighting to make it easier to read.

[seanlip]

Just a note that this issue (and #514) are a bit less straightforward than they might look on the surface. This is because they'll require forking the current MonospaceDisplayModal into two separate modals, one customized for code and the other customized for displaying output.