Reduce sample data size

[hpinkos]

Fixes #8284

• Removes unused source model files from Apps/SampleData
• Hosted the environment map for the Image Based Lighting sandcastle example
• Better compression for fire.png image used for particle system sandcastle example
• Resized PNGs for boombox test model used in specs

In total, this reduced the release zip file size from ~92MB to ~52MB

[cesium-concierge]

Thanks for the pull request @hpinkos!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[lilleyse]

Maybe keep the shadow tester models? They are used in the shadows development sandcastle. At least they aren't too big.

[hpinkos]

@lilleyse if you ever need them for testing, you can use them on your local machine. I don't see a need to package them up as part of sample data in our library.

[mramato]

I don't see anything wrong with keeping non huge models around in the repository if they are useful for debugging, but that doesn't mean they need to be part of the release zip. Perhaps have a data subdirectory or something under Development Sandcastle examples? This directory is already ignored for release so it will stop the models from being included but still have them available for testing.

[hpinkos]

Added back the shadow test models. Anything else?

[mramato]

Awesome, thanks @hpinkos!