Improvements to kubeform_spawner.py (form UI and others)

[tlkh]

Changes

1. UI
Improved spawner layout, with optional (example) bulletin showing
The bulletin was very useful at my university. It is commented out in the code by default.

For comparison, the original spawner from master

2. Some other issues in kubeform_spawner.py

• Replace double quotes (") with single quotes by default ('), if not it may cause JSON parsing errors on older installations (0.2.x tested)
• Commented out GitHubOAuthenticator since it is not being used
• Added error handling in case DEFAULT_JUPYTERLAB env variable is not found
• Added some comments on adding additonal PVC

I'm a first time contributor, please let me know if I'm doing something wrong!

On a side note, I am developing our own flavour of the Spawner UI for my university. Here's what it looks like, if there are other parts worth merging as well, please highlight!

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This change is 

[k8s-ci-robot]

Hi @tlkh. Thanks for your PR.

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[pdmack]

/hold

I'm working on some modifications to allow people to have their own spawner "skins" like this. Will consult some of the ideas here.

[jlewi]

See also discussion in #34 and #1630

[k8s-ci-robot]

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