Allow syncing some directories via Mobile Data

[mic-e]

Some of my syncthing directories are large and should only be synced when connected to an unmetered Wifi (e.g.: Photos).
Others are low-bandwidth and should be synced always (e.g.: Orgzly TODOs).

It would be nice to have fine-grained control over for which directories Syncthing runs.

[AudriusButkevicius]

Dupe of #511

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