Add a "Get involved" button to navbar

[TuringTux]

Closes #41.

To Do

• [x] The button is not vertically aligned with the other navbar entries
• [ ] The code could probably be written more generally
• [ ] The text of the button contains   because it is otherwise line-broken in an ugly way

[TuringTux]

I think this is now ready for review: Two of the "To Do" points are still open, but both are more about questions of coding style and aesthetics, which I alone cannot judge: The result (i.e. the button) is now there and it works, even if the code could maybe written more generally.

I'd be happy to hear your opinion on that :)

[TuringTux]

Thank you for the review and the nice words, ready for merge :)