New image link format

[jprochazk]

What

• Fixes Improve upload-image artifact discoverability #2461

The new link format is https://static.rerun.io/{original_image_name_minus_extension}/{content_hash}/{width}.{ext}

For example, https://static.rerun.io/ae4b8970edba8480431cb71e57b8cddd9e1769c7_clock_480w.png is now https://static.rerun.io/clock/ae4b8970edba8480431cb71e57b8cddd9e1769c7/480w.png.

What makes this more discoverable:

• For machines, it's the fact that the hash is the same for all sizes. This means given an image URL, you can retrieve all the other sizes. This is important for Docs codegen #3445.
• For humans, the top-level directory is the name of the image, which makes it much easier to browse through GCS looking for that image. You can compare the updated at date of each hash to find the latest one.

upload_image.py has been updated to use the new format. We're nowhere near the point of using too much space on GCS, so all the old links are preserved.

This PR also fixes some other issues:

• thumbnails.py did not handle updating thumbnail_dimensions correctly if the property was already present in the frontmatter
• upload_image.py would fail if the user does not have delete permission when uploading the same image twice, because it would attempt to overwrite the existing image. if the content hash does not change, then we do not need to upload the image again, so we just skip uploading to GCS if the object already exists.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested demo.rerun.io (if applicable)

• PR Build Summary
• Docs preview
• Examples preview
• Recent benchmark results
• Wasm size tracking

[jprochazk]

Linkinator is useful for once: All the new links are confirmed to work 😄

https://github.com/rerun-io/rerun/actions/runs/6313004017/job/17140171704?pr=3477

[emilk]

niiice