Improve lightbox loading indicator

[seancolsen]

Fixes item 23 in our Files UI checklist.

⚠️ Don't merge until base branch becomes file_attachment_feature ⚠️

Notes

• I have a TODO_FILES_UI comment here here because I couldn't (quickly) figure out how to work with the new colors system to get this color defined correctly. I've added item 39 to track this small task.
  • See comment.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the develop branch of the repository
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.

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[pavish]

@seancolsen I pushed a commit ede89df that fixes item 39 and removes the TODO_FILES_UI comment here.

[seancolsen]

Thanks @pavish!

[zackkrida]

This looks great!