Add a link to contribute translations to the localisation drop-down.

An attempt to solve #59.

Adds an item to the bottom of the localisation drop-down with a link to https://wiki.diasporafoundation.org/Contribute_translations

The text in the drop-down reads 'Translate it!', which isn't ideal, but I can't think of a better way to express it in a short enough form to fit into the drop-down. Any ideas are welcome.

Tested locally and seems fine, but I'd value guidance on my coding.