New notifications system in playing interface

This adds [bootstrap-growl](https://github.com/ifightcrime/bootstrap-growl) as another submodule, so another `git submodule update` may be necessary. (I considered changing the submodules to subtrees, which don't require the submodule initializing/updating, but I'm not sure if they're as easy to update.)
(There's also an unrelated commit that added a link to the admin navbar, which I forgot to include in my last PR.)
Screenshot:
![screen shot 2013-12-07 at 11 46 24 am](https://f.cloud.github.com/assets/3719547/1698710/27346fdc-5f5f-11e3-9e51-19592a37beda.png)
(these appear in the same location as the old notifications)