Factor out function to get the current user

The same logic was duplicated in three places. Factor it
out so that we can add further fallbacks in a single place.

modules/user/user.go

@@ -0,0 +1,18 @@
+// Copyright 2014 The Gogs Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package user
+
+import (
+       "os"
+)
+
+func CurrentUsername() string {
+	curUserName := os.Getenv("USER")
+	if len(curUserName) > 0 {
+		return curUserName
+	}
+
+	return os.Getenv("USERNAME")
+}