Protect against unknown emojis in frequently used list

What does this MR do?

• Protect against unknown emojis in frequently used list
  • Show grey_question ❔ when asked to render an unknown emoji
  • Strip any unknown emojis from the frequently used emoji list before being rendered

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

People can have non-standard unknown emojis set in their frequently_used_emojis cookie that will throw a JS error.

See https://gitlab.slack.com/archives/frontend/p1489690607738864

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?