Add custom emoji for groups

Follow up to https://gitlab.com/gitlab-org/gitlab-ce/issues/50581, so that you can now fully add custom emoji from groups, because this issue will implement a Web UI to do so.

Problem

This issue makes it so custom emoji are possible to be made/removed.

Scope

• Upload custom emoji from top-level group scope
  • Any group member can add custom emoji (meaning that this option should be made accessible to any group member)
    • Group members can only delete their own emoji
    • Maintainers can delete any custom emoji
    • Name and avatars of people who added custom emoji are shown with the custom emoji
    • When removal or deletion of member account happens, custom emoji will keep existing
  • Validations to make sure that the emoji labels to not collide with existing ones
  • Validations for reasonable-sized images
  • Group transfers will not bring emoji along (future improvement)
• The custom emoji will be available to all places in the group instance where emoji are entered
  • Emoji auto completer
  • Emoji menu
  • Emoji reaction search
• Geo / object storage support
• Performant emoji picker per-group

Design

• Makes it available to all group members with a customization action in the left sidebar outside of settings

Validation:

• When adding new emoji, checking for name collisions in ancestor groups and descendent groups

If member left group/is removed:

• Show the user if they still exist, otherwise move this to the ghost user: https://docs.gitlab.com/ee/user/profile/account/delete_account.html#associated-records

Tranferring groups:

• will not transfer custom emoji in the beginning (we may support this at a later stage)