Spritelab set background as block

[JillianK]

This adds a new set background as block to replace the existing set background block. In this version, the dropdown has the images corresponding to the backgrounds and you can add more from the animation library. It also changes backgrounds to be treated like animations, similar to gamelab. This should not change the behavior for animations in gamelab.

This PR is set up for testing purposes. Before merging we need to:

• Merge this PR
• remove the logic for the custom spritelab manifest along with the custom spritelab manifest (customSpritelabCostumeLibrary.json)
• Update the actual spritelab manifest in s3 to contain backgrounds
• Update default sprites to have backgrounds in them instead of the fake elements added for testing purposes (the technical changes for this are in this PR)

Links

• spec
• jira
• PR that these changes depend on

Testing story

There are tests for displaying the correct list items when we're looking for animations in spritelab, looking for animations in gamelab, and looking for backgrounds.

Reviewer Checklist:

• Tests provide adequate coverage
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[maddiedierker]

🎉 🎉 🎉