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/path_test.go

@@ -207,6 +207,6 @@ func TestCleanPath(t *testing.T) {
 		}
 	}
 	for _, c := range cases {
-		assert.Equal(t, c.expected, FilePathJoinAbs(c.elems...), "case: %v", c.elems)
+		assert.Equal(t, c.expected, FilePathJoinAbs(c.elems[0], c.elems[1:]...), "case: %v", c.elems)
 	}
 }