add link to contribute and promote pages in personal settings

This requires a »Promote« section to be added and linkable to the »Contribute« page http://owncloud.org/contribute/#promote cc @tomneedham 

The section will not appear for branded ownClouds.

Please check @karlitschek @jospoortvliet – same thing as for the firstrunwizard at https://github.com/owncloud/firstrunwizard/pull/8
No just owncloud.org/promote