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.

models/avatars/avatar_test.go

@@ -0,0 +1,54 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package avatars
+
+import (
+	"net/url"
+	"testing"
+
+	"code.gitea.io/gitea/modules/setting"
+
+	"github.com/stretchr/testify/assert"
+)
+
+const gravatarSource = "https://secure.gravatar.com/avatar/"
+
+func disableGravatar() {
+	setting.EnableFederatedAvatar = false
+	setting.LibravatarService = nil
+	setting.DisableGravatar = true
+}
+
+func enableGravatar(t *testing.T) {
+	setting.DisableGravatar = false
+	var err error
+	setting.GravatarSourceURL, err = url.Parse(gravatarSource)
+	assert.NoError(t, err)
+}
+
+func TestHashEmail(t *testing.T) {
+	assert.Equal(t,
+		"d41d8cd98f00b204e9800998ecf8427e",
+		HashEmail(""),
+	)
+	assert.Equal(t,
+		"353cbad9b58e69c96154ad99f92bedc7",
+		HashEmail("gitea@example.com"),
+	)
+}
+
+func TestSizedAvatarLink(t *testing.T) {
+	setting.AppSubURL = "/testsuburl"
+
+	disableGravatar()
+	assert.Equal(t, "/testsuburl/assets/img/avatar_default.png",
+		GenerateEmailAvatarFastLink("gitea@example.com", 100))
+
+	enableGravatar(t)
+	assert.Equal(t,
+		"https://secure.gravatar.com/avatar/353cbad9b58e69c96154ad99f92bedc7?d=identicon&s=100",
+		GenerateEmailAvatarFastLink("gitea@example.com", 100),
+	)
+}