Fix loading Web IDE Beta outside gitlab_url

What does this MR do and why?

Fix loading Web IDE Beta outside gitlab_url (see originating thread in Web IDE Feedback issue).

• Previously we depended on gitlab_url which meant that the Web IDE would only work if the browser accessing the Web IDE visited the same origin/URL ans gitlab_url.
• Now we just depend on the browser's location.origin which should make us a bit more flexible

Changelog: fixed

Screenshots or screen recordings

Before	After
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How to set up and validate locally

1. Make sure the vscode_web_ide feature flag is enabled
2. Visit the Web IDE in the local GDK instance
3. In the browser URL, replace the local GDK's configured hostname with something else that still resolves to the GDK instance (for example, 127.0.0.1 or localhost)
4. Confirm that the Web IDE still loads the file tree 👍

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.