LFS: make HTTP auth period configurable (#4035)

* LFS: make HTTP auth period configurable

* Formatting: Removed semicolon

Due to automated fmt-check failure (drone.gitea.io)

* applying code reviews

* Applied code review comment: Change HTTPAuthExpiry to time.Duration

* Updated config cheat sheet

cmd/serv.go

@@ -268,7 +268,7 @@ func runServ(c *cli.Context) error {
 		claims := jwt.MapClaims{
 			"repo": repo.ID,
 			"op":   lfsVerb,
-			"exp":  now.Add(5 * time.Minute).Unix(),
+			"exp":  now.Add(setting.LFS.HTTPAuthExpiry).Unix(),
 			"nbf":  now.Unix(),
 		}
 		if user != nil {