Refactor system setting (#27000)

This PR reduces the complexity of the system setting system.

It only needs one line to introduce a new option, and the option can be
used anywhere out-of-box.

It is still high-performant (and more performant) because the config
values are cached in the config system.

modules/repository/commits_test.go

@@ -12,7 +12,6 @@ import (
 
 	"code.gitea.io/gitea/models/db"
 	repo_model "code.gitea.io/gitea/models/repo"
-	system_model "code.gitea.io/gitea/models/system"
 	"code.gitea.io/gitea/models/unittest"
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/setting"
@@ -103,12 +102,6 @@ func TestPushCommits_ToAPIPayloadCommits(t *testing.T) {
 	assert.EqualValues(t, []string{"readme.md"}, headCommit.Modified)
 }
 
-func initGravatarSource(t *testing.T) {
-	setting.GravatarSource = "https://secure.gravatar.com/avatar"
-	err := system_model.Init(db.DefaultContext)
-	assert.NoError(t, err)
-}
-
 func TestPushCommits_AvatarLink(t *testing.T) {
 	assert.NoError(t, unittest.PrepareTestDatabase())
 
@@ -132,7 +125,7 @@ func TestPushCommits_AvatarLink(t *testing.T) {
 		},
 	}
 
-	initGravatarSource(t)
+	setting.GravatarSource = "https://secure.gravatar.com/avatar"
 
 	assert.Equal(t,
 		"https://secure.gravatar.com/avatar/ab53a2911ddf9b4817ac01ddcd3d975f?d=identicon&s="+strconv.Itoa(28*setting.Avatar.RenderedSizeFactor),