Provide a feedback button in the interface

Meant to resolve #1653. As per @dwhly's suggestion the dropdown menu is now accessible even when not logged in. Note: items relevant to only logged in users are not shown.

It now looks like this:
![hypothesis](https://cloud.githubusercontent.com/assets/521978/5252335/897f3188-7951-11e4-94ba-f8861f6e2054.png)

The feedback link consists of `mailto:support@hypothes.is` which the hypothes.is team should now have access to.