Fix "Download Meshes" functionality

[daniel-wer]

Avoid that zip.js dynamically creates a web worker using new Blob(...) which violates the CSP.
The z-worker.js seems to be requested for each mesh 🤷 but it's cached so it shouldn't be an issue.

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• Load a couple of meshes
• Use "Download Meshes" in the context menu of a group in the segments tab (this fails on master due to CSP)
• Check that the single mesh download still works

---

(Please delete unneeded items, merge only when none are left open)

• Updated changelog
• Removed dev-only changes like prints and application.conf edits
• Considered common edge cases

[philippotto]

Thanks for fixing this! 🙏 Changes LGTM. I didn't test it (CI is not ready yet), but I trust that you did so and wanted to leave an approval already.

[dieknolle3333]

works great for me, thanks for looking into this :)