Avatar refactor, move avatar code from `models` to `models.avatars`, remove duplicate...

Avatar refactor, move avatar code from `models` to `models.avatars`, remove duplicated code (#17123)

Why this refactor

The goal is to move most files from `models` package to `models.xxx` package. Many models depend on avatar model, so just move this first.

And the existing logic is not clear, there are too many function like `AvatarLink`, `RelAvatarLink`, `SizedRelAvatarLink`, `SizedAvatarLink`, `MakeFinalAvatarURL`, `HashedAvatarLink`, etc. This refactor make everything clear:

* user.AvatarLink()
* user.AvatarLinkWithSize(size)
* avatars.GenerateEmailAvatarFastLink(email, size)
* avatars.GenerateEmailAvatarFinalLink(email, size)

And many duplicated code are deleted in route handler, the handler and the model share the same avatar logic now.