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>

modules/util/timer.go

@@ -4,6 +4,7 @@
 package util
 
 import (
+	"sync"
 	"time"
 )
 
@@ -18,3 +19,30 @@ func StopTimer(t *time.Timer) bool {
 	}
 	return stopped
 }
+
+func Debounce(d time.Duration) func(f func()) {
+	type debouncer struct {
+		mu sync.Mutex
+		t  *time.Timer
+	}
+	db := &debouncer{}
+
+	return func(f func()) {
+		db.mu.Lock()
+		defer db.mu.Unlock()
+
+		if db.t != nil {
+			db.t.Stop()
+		}
+		var trigger *time.Timer
+		trigger = time.AfterFunc(d, func() {
+			db.mu.Lock()
+			defer db.mu.Unlock()
+			if trigger == db.t {
+				f()
+				db.t = nil
+			}
+		})
+		db.t = trigger
+	}
+}