api: Add go files

I know this is a huge commit, but I can't be bothered to check
this in part by part.

api/oauth_google_redirect.go

@@ -0,0 +1,24 @@
+package main
+
+import (
+	"fmt"
+	"net/http"
+)
+
+func googleRedirectHandler(w http.ResponseWriter, r *http.Request) {
+	session := r.FormValue("session")
+
+	c, err := commenterGetBySession(session)
+	if err != nil {
+		fmt.Fprintf(w, "error: %s\n", err.Error())
+		return
+	}
+
+	if c.CommenterHex != "none" {
+		fmt.Fprintf(w, "error: that session is already in use\n")
+		return
+	}
+
+	url := googleConfig.AuthCodeURL(session)
+	http.Redirect(w, r, url, http.StatusFound)
+}