Refactor embedded assets and drop unnecessary dependencies (#34692)

Benefits:

1. smaller binary size (reduces more than 1MB)
2. better control of the assets details
3. fewer unmaintained dependencies
4. faster startup if the assets are not needed
5. won't hang up editors when open "bindata.go" by accident

modules/templates/templates_dynamic.go

@@ -0,0 +1,15 @@
+// Copyright 2016 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build !bindata
+
+package templates
+
+import (
+	"code.gitea.io/gitea/modules/assetfs"
+	"code.gitea.io/gitea/modules/setting"
+)
+
+func BuiltinAssets() *assetfs.Layer {
+	return assetfs.Local("builtin(static)", setting.StaticRootPath, "templates")
+}