Add extension upgrade script

[blink1073]

References

Fixes #8870
Fixes #8869
Relies on jupyterlab/jupyterlab_server#123

Code changes

• Adds a script for extension authors python -m jupyterlab.upgrade_extension to automatically upgrade their scripts.
  They are prompted before files are overwritten.
• Fixes handling of watch mode and makes watch mode easier for extension authors

User-facing changes

Easier for extension authors to upgrade their scripts.

Backwards-incompatible changes

None

[jupyterlab-dev-mode]

Thanks for making a pull request to JupyterLab!

To try out this branch on binder, follow this link:

[blink1073]

TODO: we need to move all handling of page_config to jupyterlab_server, so we get the latest page_config in the handler.

[blink1073]

One more remaining TODO that I need to sync up with @Zsailer on today, otherwise this is entirely working for the upgraded example in jupyterlab/extension-examples#113 in watch mode.

[blink1073]

I am cutting a beta with this.