show user review tips when low app rating is given (bug 874947)

https://bugzilla.mozilla.org/show_bug.cgi?id=874947

# 4 stars or higher
![](http://f.cl.ly/items/3e2Z2U000S1i1x0c1M2S/Screen%20Shot%202014-06-09%20at%2010.53.15%20AM.png)

# 3 stars or lower
![](http://cl.ly/image/2G3735172Y3h/Screen%20Shot%202014-06-06%20at%2010.12.52%20AM.png)
![](http://cl.ly/image/2p3j183d1d1o/Screen%20Shot%202014-06-06%20at%2010.14.29%20AM.png)
![](http://cl.ly/image/3f10411k3s1Y/Screen%20Shot%202014-06-06%20at%2010.17.53%20AM.png)

this is hacky. this should actually look up the app in the model cache, since the app object is guaranteed to be there.