Warning about FooterLink keys in dev

There is a warning in the browser console when running the project in development mode. The warning can be fixed by adding a key prop to FooterLink elements here and here. If this should be fixed I will submit a PR to do so.

Warning text:

index.js:2177 Warning: Each child in an array or iterator should have a unique "key" prop.

Check the render method of `Footer`. See https://fb.me/react-warning-keys for more information.
    in FooterLink (at Footer.js:67)
    in Footer (at index.js:69)
    in div (at index.js:45)
    in Template (at index.js:6)
    in _default (created by ComponentRenderer)
    in ComponentRenderer (created by Route)
    in Route (created by withRouter(ComponentRenderer))
    in withRouter(ComponentRenderer) (created by Root)
    in ScrollContext (created by Route)
    in Route (created by withRouter(ScrollContext))
    in withRouter(ScrollContext) (created by Root)
    in Router (at root.js:143)
    in DefaultRouter (created by Root)
    in Root (at app.js:45)
    in AppContainer (at app.js:44)

Steps to reproduce:

1. Checkout master branch
2. Run yarn dev
3. Navigate to localhost:8000 and look in the browser console

Browser: Chrome 63
OS: MacOS 10.13.1

Öffne src/components/LayoutFooter/Footer.js bei den FooterLink-Elementen in den Zeilen 67 und 115, führe anschließend yarn dev aus und rufe localhost:8000 auf. Bestätige, dass die Browserkonsole die Warnung über den fehlenden eindeutigen Schlüssel nicht mehr ausgibt.