Allow setting X-FRAME-OPTIONS (#16643)

* Allow setting X-FRAME-OPTIONS

This PR provides a mechanism to set the X-FRAME-OPTIONS header.

Fix #7951

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Update docs/content/doc/advanced/config-cheat-sheet.en-us.md

Co-authored-by: John Olheiser <john.olheiser@gmail.com>

Co-authored-by: John Olheiser <john.olheiser@gmail.com>

modules/setting/cors.go

@@ -20,9 +20,11 @@ var (
 		Methods          []string
 		MaxAge           time.Duration
 		AllowCredentials bool
+		XFrameOptions    string
 	}{
-		Enabled: false,
-		MaxAge:  10 * time.Minute,
+		Enabled:       false,
+		MaxAge:        10 * time.Minute,
+		XFrameOptions: "SAMEORIGIN",
 	}
 )