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_test.go

@@ -0,0 +1,30 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package util
+
+import (
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDebounce(t *testing.T) {
+	var c int64
+	d := Debounce(50 * time.Millisecond)
+	d(func() { atomic.AddInt64(&c, 1) })
+	assert.EqualValues(t, 0, atomic.LoadInt64(&c))
+	d(func() { atomic.AddInt64(&c, 1) })
+	d(func() { atomic.AddInt64(&c, 1) })
+	time.Sleep(100 * time.Millisecond)
+	assert.EqualValues(t, 1, atomic.LoadInt64(&c))
+	d(func() { atomic.AddInt64(&c, 1) })
+	assert.EqualValues(t, 1, atomic.LoadInt64(&c))
+	d(func() { atomic.AddInt64(&c, 1) })
+	d(func() { atomic.AddInt64(&c, 1) })
+	d(func() { atomic.AddInt64(&c, 1) })
+	time.Sleep(100 * time.Millisecond)
+	assert.EqualValues(t, 2, atomic.LoadInt64(&c))
+}