Disable remote thumbnailing on metered networks

Assuming #941 is resolved by turning thumnails on by default: on metered networks (which network manager knows about), we should turn remote thumbnails off but have an infobar telling the user why it's off and offering to turn it back on.

Network manager automatically sets the metered network flag on Android hotspots, and users can set it manually on networks as well. This will become especially important if https://github.com/elementary/switchboard-plug-network/issues/165 is implemented.

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