Make Theia WebViews more secure

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At the moment, Theia WebViews are on the same origin as Theia editor. That's not secure as WebViews have access to the Theia page and some plugin can break Theia.

Theia WebViews should have a separate origin to be isolated from Theia page.

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The related draft PRs to track:

• Ingress/Route per plugin endpoint Ingress/Route per plugin endpoint #15432
• Use a dedicated server as WebView domain Use a dedicated server as webview domain che-theia#591
• Add Che Theia Editor endpoint to expose the plugins' WebViews on a different origin to make them isolated from Che Theia origin Add Che Theia editor endpoint to expose the plugins' webviews on a different origin to make them isolated from Che Theia origin che-plugin-registry#335