[FIX JENKINS-23041] Fix systemInfo for offline slaves

- Don't show link to `systemInfo` in sidebar when slave is offline
- Make `systemInfo` page more robust when accessed via URL by not trying to show anything

Also, nicer page title: Show icon, name, and description of slave at the top of the page.

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Are there `SlaveSystemInfo` that show real information while slave is offline? Maybe the robustness actually needs to be moved into the individual `SlaveSystemInfo`?

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![screen shot 2014-05-15 at 05 11 11](https://cloud.githubusercontent.com/assets/1831569/2980061/25e2f69c-dbdf-11e3-9897-cdd44576053b.png)

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![screen shot 2014-05-15 at 05 10 57](https://cloud.githubusercontent.com/assets/1831569/2980062/27e0e9ae-dbdf-11e3-8762-c273dd95407c.png)