Validates that user is a human.

Add google recaptcha as an initial page before creating any session.
To configure recaptcha there are 2 environment variables that are needed
`GOOGLE_RECAPTCHA_SITE_KEY` and `GOOGLE_RECAPTCHA_SITE_SECRET`.
The code contains development defaults that should be set in production
to real values.
**NOTICE: Development defaults assume that the domain is `localhost`**

services/recaptcha.go

@@ -0,0 +1,59 @@
+package services
+
+import (
+	"encoding/json"
+	"log"
+	"net/http"
+	"net/url"
+	"os"
+	"strings"
+)
+
+func GetGoogleRecaptchaSiteKey() string {
+	key := os.Getenv("GOOGLE_RECAPTCHA_SITE_KEY")
+	if key == "" {
+		// This is a development default. The environment variable should always be set in production.
+		key = "6LeY_QsUAAAAAOlpVw4MhoLEr50h-dM80oz6M2AX"
+	}
+	return key
+}
+func GetGoogleRecaptchaSiteSecret() string {
+	key := os.Getenv("GOOGLE_RECAPTCHA_SITE_SECRET")
+	if key == "" {
+		// This is a development default. The environment variable should always be set in production.
+		key = "6LeY_QsUAAAAAHIALCtm0GKfk-UhtXoyJKarnRV8"
+	}
+
+	return key
+}
+
+type recaptchaResponse struct {
+	Success bool `json:"success"`
+}
+
+func IsHuman(req *http.Request) bool {
+	req.ParseForm()
+	challenge := req.Form.Get("g-recaptcha-response")
+
+	// Of X-Forwarded-For exists, it means we are behind a loadbalancer and we should use the real IP address of the user
+	ip := req.Header.Get("X-Forwarded-For")
+	if ip == "" {
+		// Use the standard remote IP address of the request
+
+		ip = req.RemoteAddr
+	}
+
+	parts := strings.Split(ip, ":")
+
+	resp, postErr := http.PostForm("https://www.google.com/recaptcha/api/siteverify", url.Values{"secret": {GetGoogleRecaptchaSiteSecret()}, "response": {challenge}, "remoteip": {parts[0]}})
+	if postErr != nil {
+		log.Println(postErr)
+		// If there is a problem to connect to google, assume the user is a human so we don't block real users because of technical issues
+		return true
+	}
+
+	var r recaptchaResponse
+	json.NewDecoder(resp.Body).Decode(&r)
+
+	return r.Success
+}