This adds the `emoji_picker_pinned_groups` site setting for pinning
emoji groups at the top of the emoji picker list. It accepts both custom
& non custom emoji group names, and can be ordered to preference.
If a custom emoji group is added, and the group is deleted, it will
gracefully ignore the missing group. A validator is included to ensure
the site setting is correct at time of save.
<img width="912" height="228" alt="image"
src="https://github.com/user-attachments/assets/0a832cd9-146d-4d46-b9d1-4e8d4ea3b30d"
/>
<img width="464" height="354" alt="image"
src="https://github.com/user-attachments/assets/2512df0e-035c-457a-bed3-b5d0ac3edc2c"
/>
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Co-authored-by: Martin Brennan <martin@discourse.org>