use flexbox to scale the list size of widgets in the variable layout

Thanks for the awesome new tab page! When i switched to the "grid" layout, I noticed that I could extend the height of the twitter widget, but that the list didn't adjust in size appropriately (see the screenshots below). 

When I went looking through the css I updated several other widgets to have the same behavior, but was not able to verify that all of them work as expected.

**Before:**
![image](https://cloud.githubusercontent.com/assets/1329312/5034655/f49245d2-6b2d-11e4-9edc-7aec05976cb0.png)

**After:**
![image](https://cloud.githubusercontent.com/assets/1329312/5034647/ea4a768a-6b2d-11e4-9102-5d5cad1d74f3.png)