[SAGECell] Remove online/offline tag

[mlr77]

• Kernel dropped but SAGECell was not updated — no way to check until execution time without pinging the server every few seconds.
• Maybe the better option is to remove the online/offline tag altogether and just let the user know if the kernel died when it tries to execute, if possible.
• Once the kernel dashboard is moved to the panel and has its own collection, it should update the list of kernels available. Then SAGECell can also update its local kernel list using a useEffect to watch for changes in the collection.

[renambot]

• I think we want more of the online/offline light

[mlr77]

• I think we want more of the online/offline light

Okay, I think I got it finally. I was able to get it all working in the Seer app to test it out

In the Seer app, I added two status spots, one in the toolbar, and one in the app window. The toolbar shows:

• Online (yellow) if there is a kernel but just isn't selected
• Ready (green) if it's selected
• Locked (red) if the user has it locked because they're using a private kernel
• Locked by ownerName (red) if it's not a public/shared kernel
• Offline (red) if there are no kernels running The app shows either Online (green), Offline (red) or No Access (red)

I'll try to do a PR for that app so we can test if it's working okay and decide whether or not to add any similar features to SAGECell.