Stricter requirements to consider a client healthy

A healthy client must not only have a sync task running, but also have
finished its initial sync and have been succesful in its last sync
attempt.

The previous weak requirement caused this issue:
Have two clients, one on a failing server, and try to fetch a media,
thumbnail or profile: since these functions don't need auth, any
healthy client is picked to do the request, but the previous
requirement made it possible to pick the failing client.

src/icons/thin/image-rotate-left.svg

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src/icons/thin/image-rotate-right.svg

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