Receive notifications on emoji reactions on comments in issue/merge request/epic

Description

This is when a user adds an emoji reaction to a comment. This is different from when a user adds an emoji on the object itself (below the description). This is tracked in https://gitlab.com/gitlab-org/gitlab-ce/issues/54152.

Original description

Description

Team Member A creates an Issue or Merge Request, and Team Member B validates it with a Thumbs Up. Team Member A does not know about the reaction, until they revisit the Issue or Merge Request on their own.

Proposal

Emoji reactions should send notifications, just like Approval or Comments.

Links / references