Add Save button to top of user preferences

This adds a Save button to the top of the User Preferences Page.
[See related thread on meta.](https://meta.discourse.org/t/changes-in-profile-preferences-dont-get-saved-automatically/12198/6)

*Desktop:*
![image](https://f.cloud.github.com/assets/3401659/2198256/e80fd03a-98c5-11e3-83c1-382e79c316df.png)

*Mobile:*
![image](https://f.cloud.github.com/assets/3401659/2198276/46bdb930-98c6-11e3-9dd9-594db6f3a7ef.png)