add a fallback font stack for emojis

[SallyMcGrath]

What does this change?

Common Theme?

adds an explicit fallback stack of emoji fonts
then assigns that to c.emoji

a gesture towards
Issue number #867

Checklist

• I have read the contributing guidelines
• I have checked my spelling and grammar with an automated tool
• I have previewed my changes to check the markdown renders as I intend
• I have run my code to check it works
• My changes follow our Style Guide

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