Add vanilla JS avatar_helper and update existing avatar helpers

What does this MR do?

• Adds vanillas JS avatar_helper
• Updates existing avatar helpers

(This came from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20332/diffs, but there was some reuse identified, so it's been pulled into it's own MR.)

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

Why was this MR needed?

• #47865 (moved)
• @dennis
• https://gitlab.com/gitlab-org/gitlab-ce/issues/36409

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • [ ] Has been reviewed by a Backend maintainer
  • [ ] Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

• #47865 (moved)