Fixing flashes. The previous implementation is both wrong and bad

1. It was wrong because the syntax was being misused. `#a#{b}` is
   telling slim to give two `id` attributes, and not to interpolate the
   `b` variable. And giving two id attributes throws an error. So
   firestarter apps wouldn't even run out of the box

2. Even with the correct syntax, we would be giving ids based on the
   flash key, which might create duplicates. We should use classes
   anyway