+ [devdoc] Add a way for block developers to get and save system settings that persist across all requests.

Example...

        int requestCount = 0;
        if (!int.TryParse(Rock.Web.SystemSettings.GetValue("StarkRequestCount"), out requestCount))
        {
            requestCount = 0;
        }
        requestCount++;
        Rock.Web.SystemSettings.SetValue("StarkRequestCount", requestCount.ToString(), null);

        lRequestCount.Text = requestCount.ToString();