Verify stored search settings are valid before defaulting to them

[tiffon]

I sometimes have an issue where I bring up empty Jaeger UI (e.g. from a fresh all-in-one Docker image), so the Services dropdown is supposed to be empty, but the screen renders with a value from the previous runs on the same URL. I know we store the previously selected service name in local storage, but when we retrieve it we should probably check if the value is actually in the list of currently available services.