Use a general approach to access custom/static/builtin assets (#24022)

The idea is to use a Layered Asset File-system (modules/assetfs/layered.go)

For example: when there are 2 layers: "custom", "builtin", when access
to asset "my/page.tmpl", the Layered Asset File-system will first try to
use "custom" assets, if not found, then use "builtin" assets.

This approach will hugely simplify a lot of code, make them testable.

Other changes:

* Simplify the AssetsHandlerFunc code
* Simplify the `gitea embedded` sub-command code

---------

Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

routers/install/setting.go

@@ -30,7 +30,7 @@ func PreloadSettings(ctx context.Context) bool {
 		}
 
 		setting.LoadSettingsForInstall()
-		svg.Init()
+		_ = svg.Init()
 	}
 
 	return !setting.InstallLock
@@ -47,6 +47,5 @@ func reloadSettings(ctx context.Context) {
 		} else {
 			log.Fatal("ORM engine initialization failed: %v", err)
 		}
-		svg.Init()
 	}
 }