Use relative paths for group/project/user avatars

What does this MR do?

This MR changes these methods:

1. Group#avatar_url
2. Project#avatar_url
3. User#avatar_url

Instead of using full path for avatars all the time, now we respect asset_host configuration.
It allows to use relative paths if asset_host is set to nil, and load avatars from a standalone assets server if asset_host is set.

Why was this MR needed?

Project avatars are not loaded properly if:

1. GitLab works behind proxy servers.
2. a user uses a standalone server to manage assets.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #13418 (closed) and #19662 (closed)