AllowEditPost and PostEditTimeLimit migration (#8208)

* AllowEditPost and PostEditTimeLimit migration

* Not set EDIT_POST permission to sysadmin_role if ALLOW_EDIT_POST is configured to NEVER

* Remove a bit of code duplication

model/config.go

@@ -419,7 +419,7 @@ func (s *ServiceSettings) SetDefaults() {
 	}
 
 	if s.PostEditTimeLimit == nil {
-		s.PostEditTimeLimit = NewInt(300)
+		s.PostEditTimeLimit = NewInt(-1)
 	}
 
 	if s.EnablePreviewFeatures == nil {