Standardize modal hide/show logic to support transition animations

[twschiller]

Context

• https://github.com/pixiebrix/pixiebrix-extension/issues/2246#issuecomment-1128240540
• Modifying these will require some verification - by moving the flag to the show property, the we might hit null pointer errors if the code was guarding against raw/unitialized state

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This issue will be closed in 7 days unless the stale label is removed, or a comment is added to the issue.

[github-actions[bot]]

This issue will be closed in 7 days unless the stale label is removed, or a comment is added to the issue.

[github-actions[bot]]

This issue was closed because it has been stale for 7 days with no activity.