Add "Registry" to the Explore menu

Purpose
-----------
OSF is a registry. People do not understand this. Having a registry link is a quick first step to making that clear. 

Changes
------------
Adds a menu item to the Explore menu called Registry that takes the user to a Search page of all of the OSF registrations.

![screen shot 2014-12-04 at 10 50 29 am](https://cloud.githubusercontent.com/assets/6599111/5301104/efe659d4-7ba3-11e4-87d5-621463d00839.png)


Side effects
----------------
Rearranged the Explore menu to put registry at the top and Collaborator Network, which becomes increasingly out of date, at the bottom.

Closes #1331