add "Get Involved" link to desktop footer and mobile Account Settings (bug 1001208)

see https://bugzilla.mozilla.org/show_bug.cgi?id=1001208#c30

related: #795

![screenshot 2014-11-13 18 35 08](https://cloud.githubusercontent.com/assets/203725/5040486/5d443474-6b64-11e4-874d-1b456100ce0d.png)