Re-add GPG configuration feature (#374)

This reverts d5ce1a47ea and therefore adds the GPG feature back into main.
As it is a breaking change, this PR now also contains the required upgrade notes.

Closes #107 again.

Co-authored-by: justusbunsi <sk.bunsenbrenner@gmail.com>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/374
Reviewed-by: pat-s <pat-s@noreply.gitea.io>
Reviewed-by: John Olheiser <john+gitea@jolheiser.com>

CONTRIBUTING.md

@@ -50,3 +50,13 @@ be used:
    forwarded first from `minikube` to localhost first via `kubectl --namespace
    default port-forward svc/gitea-http 3000:3000`. Now Gitea is accessible at
    [http://localhost:3000](http://localhost:3000).
+
+### Unit tests
+
+```bash
+# install the unittest plugin
+$ helm plugin install https://github.com/heyhabito/helm-unittest
+
+# run the unittests
+make unittests
+```