#8759: always show mod variable data panel in the Page Editor

[twschiller]

What does this PR do?

• Closes #8759
• Always show the mod variable data panel in the Page Editor
• Rename tab name in data panel from Page State to "Mod Variables"
• Cleans up some outdated/ambiguous tab and React component names

Coordination

• Changing the tab name/event keys might impact @fungairino's POM work

Checklist

Leave all that are relevant and check off as completed

• [x] This PR requires a documentation change (link to old docs):
  • [ ] https://docs.pixiebrix.com/developing-mods/developer-concepts/data-context/using-mod-variables

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