Make "Do not mark messages read on scroll" option findable

We have a "Do not mark messages read on scroll" setting, which can be quite useful for certain workflows. At present, this setting is hidden in Gear menu > Diagnostics > Debug.

To make it discoverable, we should move this setting to the top level of the gear menu. It can be placed right below the "Open links with in-app browser" option.

We should fix #4849 before making this setting finable.