Issue description updates don't add highlighting classes

I'm going to add some mathematics and some code here:

This math is inline $`a^2+b^2=c^2`$.

```ruby
1.upto(3) do { |i| puts "#{i} time(s)" }
```

This math is inline a^2+b^2=c^2.

1.upto(3) do { |i| puts "#{i} time(s)" }

These should be rendered as mathematics, and syntax-highlighted using my chosen theme.