Update user-facing bad plurals of 'emoji' in the application

What does this MR do?

This merge request is a follow-up to clean up user-facing instances of emojis (instead of emoji) in user-facing strings and their tests. It's related to two other MRs I created today last week:

• Substitution rule: emoji does not take 's' in t... (!125778 - merged) to propose we change our guidance on the plural of emoji (AKA no more emojis)
• Clean up bad plural of 'emoji' in docs (!125782 - merged) (for the docs)

@annabeldunstone raised the question as part of Draft: Added group settings for creating custom... (!124855 - closed). I'd noticed we had a potential problem earlier in the week when I saw @iamphill's merge request about the new custom-emoji feature, but was trying to not stick my nose into EVERYthing. When it was discussed in #docs and we learned our current guidance was wrong, I realized I should step up with some corrective merge requests.

Related issues

• Discussion source: https://gitlab.slack.com/archives/C16HYA2P5/p1688744155649419 in #docs.
• Other guides:
  • https://www.grammarly.com/blog/emojis-emoji/
  • https://developers.google.com/style/word-list#letter-e
  • https://learn.microsoft.com/en-us/style-guide/a-z-word-list-term-collections/e/emoticons-emoji
• @marcel.amirault's discussion about nouns lifted from Japanese

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