Add GitHub link

There's already a link to GitHub to report issues, but I figured it's
nice to have a general link as well if that's what people are looking
for.

templates/base.html

@@ -108,6 +108,7 @@ <h1><a href="/">C++ Quiz</a></h1>
 <a href="https://mastodon.online/@cppquiz"><img src="{{STATIC_URL}}/mastodon.png" height="15" alt="Mastodon"> Mastodon</a> |
 <a href="https://bsky.app/profile/cppquiz.bsky.social"><img src="{{STATIC_URL}}/bluesky.png" height="15" alt="Bluesky"> Bluesky</a> |
 <a href="https://x.com/intent/follow?screen_name=CppQuiz" class="x-follow"><img src="{{STATIC_URL}}/x.png" height="9" alt="X"> X</a> |
+<a href="https://github.com/knatten/cppquiz"> GitHub</a> |
 © <a href="http://knatten.org">Anders Schau Knatten</a> {% now "Y"%}.
 </p>
 <p>