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

開啟 src/components/LayoutFooter/Footer.js，找到第 67 行和第 115 行的 FooterLink 元素，然後執行 yarn dev 並造訪 localhost:8000。確認瀏覽器主控台不再回報缺少唯一 key 的警告。