Initial custom emoji implementation

• Add custom emoji to markdown rendering and autocomplete.
• You can add new custom emojis via group or profile settings

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

Todo

• Add Geo support, https://gitlab.com/gitlab-org/gitlab-ee/issues/3903
• Test with relative_url_root, https://gitlab.com/gitlab-org/gitlab-ce/issues/28705

What are the relevant issue numbers?

Closes #13931 (closed)