Restructure project

We previously didn't really had any structure in our project apart
from creating a new folder for each package in our project root.
Now that we have accumulated some packages, we use the well-known
Golang project layout in order to clearly communicate our intent
with packages. See https://github.com/golang-standards/project-layout

internal/api/auth/auth.go

@@ -0,0 +1,36 @@
+package auth
+
+import (
+	"crypto/subtle"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/internal/config"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/pkg/logging"
+	"net/http"
+)
+
+var log = logging.GetLogger("api/auth")
+
+const TokenHeader = "X-Poseidon-Token"
+
+var correctAuthenticationToken []byte
+
+// InitializeAuthentication returns true iff the authentication is initialized successfully and can be used.
+func InitializeAuthentication() bool {
+	token := config.Config.Server.Token
+	if token == "" {
+		return false
+	}
+	correctAuthenticationToken = []byte(token)
+	return true
+}
+
+func HTTPAuthenticationMiddleware(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		token := r.Header.Get(TokenHeader)
+		if subtle.ConstantTimeCompare([]byte(token), correctAuthenticationToken) == 0 {
+			log.WithField("token", token).Warn("Incorrect token")
+			w.WriteHeader(http.StatusUnauthorized)
+			return
+		}
+		next.ServeHTTP(w, r)
+	})
+}