Add optional +/− controls for live canvas resizing in Preview panel

[Geethegreat]

Related to #3768

Changes:

I have verified that this pull request:

• Adds + / − resize controls to the Preview header
• Resizes the active canvas live using resizeCanvas() without reloading
• Intentionally limited to the non-logged-in editor view for safety

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123
• meets the standards outlined in the accessibility guidelines