Highlighting the currently active stream in the leftNavBar

- This gives the user the sense of what he/she is currently viewing
- The hover/selected are the same right now. Needs to change.
- Need to find a more meaningful place for `markSelected`
Moved markSelected to app.views.Stream

- Removes duplication
- All streams create this view, and this seems to do some setup on
  initializing, which is a good place to markNavSelected