Plugin manager improvements

[jtpio]

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https://github.com/jupyter/notebook/pull/7198 added the plugin manager to left area:

notebook-pluginmanager.webm

This enables the feature, which is still hidden by default. However this might not be the best in terms of UX, so we can probably iterate more on it.

Improvements to consider:

• open on the file browser page like the settings editor?
• add UI test