api: add github oauth

Closes https://gitlab.com/commento/commento/issues/20

api/oauth_github_redirect.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"fmt"
+	"net/http"
+)
+
+func githubRedirectHandler(w http.ResponseWriter, r *http.Request) {
+	if githubConfig == nil {
+		logger.Errorf("github oauth access attempt without configuration")
+		fmt.Fprintf(w, "error: this website has not configured github OAuth")
+		return
+	}
+
+	commenterToken := r.FormValue("commenterToken")
+
+	_, err := commenterGetByCommenterToken(commenterToken)
+	if err != nil && err != errorNoSuchToken {
+		fmt.Fprintf(w, "error: %s\n", err.Error())
+		return
+	}
+
+	url := githubConfig.AuthCodeURL(commenterToken)
+	http.Redirect(w, r, url, http.StatusFound)
+}