Implement "embedded" command to extract static resources (#9982)

* draft

* Implement extract command

* Fix nits and force args on extract

* Add !bindata stub, support Windows, fmt

* fix vendored flag

* Remove leading slash for matching

* Add docs

* Fix typos

* Add embedded view command

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

cmd/embedded_stub.go

@@ -0,0 +1,30 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+// +build !bindata
+
+package cmd
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/urfave/cli"
+)
+
+// Cmdembedded represents the available extract sub-command.
+var (
+	Cmdembedded = cli.Command{
+		Name:        "embedded",
+		Usage:       "Extract embedded resources",
+		Description: "A command for extracting embedded resources, like templates and images",
+		Action:      extractorNotImplemented,
+	}
+)
+
+func extractorNotImplemented(c *cli.Context) error {
+	err := fmt.Errorf("Sorry: the 'embedded' subcommand is not available in builds without bindata")
+	fmt.Fprintf(os.Stderr, "%s\n", err)
+	return err
+}