Disable thumbnails/previews if on metered data
While #133 covers the topic of whether previews should be optional and how that should be presented to the user, I think that previews should always be disabled when on metered data. Downloading pages or images can add up to a reasonable amount of bandwidth over time, and the user might not even realise to turn them off (even if they could) in the way that they might think to turn off torrents while on a metered connection.
It should be a case of adding a check against (the JS equivalent of) g_network_monitor_get_network_metered()
before calling the thumbnailer.