Admin tabs redesign

Here's another design experiment for discussion.

Having tabs horizontally at the top of the admin section means they're constrained for space. If there are more tabs than can fit in the fixed width, the rightmost ones disappear from view. A novice user would have no way of knowing those tabs exist. A carousel is available, but this makes navigation less convenient, adding extra clicks to get from tab to tab.

This pull request moves tabs to the left side of the admin area, running vertically, and removes the carousel entirely. If there are enough tabs to run off the bottom of the visible screen, they can be found with the same vertical scrolling action that practically all web users already know.

Tabs can still be manually reordered as before.

![screen shot 2014-05-16 at 2 58 16 pm](https://cloud.githubusercontent.com/assets/342659/2993087/019a0560-dca6-11e3-8582-909b26cce7c0.png)