Quest image added in the completion screen

referencing https://github.com/HabitRPG/habitrpg/issues/4333, I've added in the image (it wasn't there previously) and made it so that the reward text had to clear the image before starting. This works well because the final text can still float left of the image, but doesn't break it if there is no text. As far as the other issue that was referenced, not all quests have completion text, which is why sometimes no text appears. 
![screenshot 2014-12-11 21 56 22](https://cloud.githubusercontent.com/assets/5234453/5416320/41506626-81e3-11e4-8fc4-6a77c2c63420.png)

I try to keep inline styles to a minimum. I'd prefer this to be added [here](https://github.com/HabitRPG/habitrpg/blob/develop/public/css/quests.styl)

```
quest-rewards
    hr
        clear: both
```

That should also keep it from overflowing in non-completion quest panes with very short pieces of text.

If you don't feel up to diving into stylus, I can move it once this is merged in.