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.

templates/base/head.tmpl

@@ -48,11 +48,11 @@
 			tributeValues: Array.from(new Map([
 				{{ range .Participants }}
 				['{{.Name}}', {key: '{{.Name}} {{.FullName}}', value: '{{.Name}}',
-				name: '{{.Name}}', fullname: '{{.FullName}}', avatar: '{{.RelAvatarLink}}'}],
+				name: '{{.Name}}', fullname: '{{.FullName}}', avatar: '{{.AvatarLink}}'}],
 				{{ end }}
 				{{ range .Assignees }}
 				['{{.Name}}', {key: '{{.Name}} {{.FullName}}', value: '{{.Name}}',
-				name: '{{.Name}}', fullname: '{{.FullName}}', avatar: '{{.RelAvatarLink}}'}],
+				name: '{{.Name}}', fullname: '{{.FullName}}', avatar: '{{.AvatarLink}}'}],
 				{{ end }}
 				{{ range .MentionableTeams }}
 					['{{$.MentionableTeamsOrg}}/{{.Name}}', {key: '{{$.MentionableTeamsOrg}}/{{.Name}}', value: '{{$.MentionableTeamsOrg}}/{{.Name}}',

templates/repo/issue/view_content/comments.tmpl

@@ -746,7 +746,7 @@
 		<div class="timeline-item-group">
 			<div class="timeline-item event" id="{{.HashTag}}">
 				<a class="timeline-avatar"{{if gt .Poster.ID 0}} href="{{.Poster.HomeLink}}"{{end}}>
-					<img src="{{.Poster.RelAvatarLink}}">
+					<img src="{{.Poster.AvatarLink}}">
 				</a>
 				<span class="badge grey">{{svg "octicon-x" 16}}</span>
 				<span class="text grey">