Add option to change username to the admin panel (#14229)

Co-authored-by: Bwko <bouwko@gmail.com>
Co-authored-by: techknowlogick <matti@mdranta.net>
Co-authored-by: zeripath <art27@cantab.net>

modules/auth/admin.go

@@ -28,6 +28,7 @@ func (f *AdminCreateUserForm) Validate(ctx *macaron.Context, errs binding.Errors
 // AdminEditUserForm form for admin to create user
 type AdminEditUserForm struct {
 	LoginType               string `binding:"Required"`
+	UserName                string `binding:"AlphaDashDot;MaxSize(40)"`
 	LoginName               string
 	FullName                string `binding:"MaxSize(100)"`
 	Email                   string `binding:"Required;Email;MaxSize(254)"`