Draft: Added group settings for creating custom emoji

What does this MR do and why?

Adds the group settings page for allowing users to view, create, and delete custom emojis

Screenshots or screen recordings

How to set up and validate locally

1. Enable custom_emoji feature flag
2. Visit /groups/gitlab-org/-/custom_emojis/
3. or visit a group and click Custom emoji under Manage heading

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.