WebView: Support CSP Fix

[FernandoAscencio]

Implementation of the proposed fix for the CORS issue that did not allow images to be displayed in previews.

Sugned-off-by: FernandoAscencio [email protected]

What it does

Fix #11982 as proposed in the same article.

How to test

1. Build and start the Theia Electron example from sources. Developing.md
2. Install extension asciidoctor from extensions view open an asciidoc source file that references local images. E.g. "article.adoc" from repo [asciidoc-samples] https://github.com/opendevise/asciidoc-samples)
3. Open the preview for the source file (click the corresponding button at the top right corner of editor)
4. Confirm that images are shown in preview

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

Looks good to me 👍

• Webviews continue to work as expected
• The Asciidoc extension shows images as expected

[marcdumais-work]

Thanks @FernandoAscencio and @msujew !