Sendmail command (#13079)

* Add SendSync method

Usefull to have when you need to be confident that message was sent.

* Add sendmail command

* add checks that if either title or content is empty then error out

* Add a confirmation step

* Add --force option to bypass confirm step

* Move implementation of runSendMail to a different file

* Add copyrighting comment

* Make content optional

Print waring if it's empty or haven't been set up.
The warning will be skiped if there's a `--force` flag.

* Fix import style

Co-authored-by: 6543 <6543@obermui.de>

* Use batch when getting all users

IterateUsers uses batching by default.

Signed-off-by: Maxim Zhiburt <zhiburt@gmail.com>

* Send emails one by one instead of as one chunck

Signed-off-by: Maxim Zhiburt <zhiburt@gmail.com>

* Send messages concurantly

Signed-off-by: Maxim Zhiburt <zhiburt@gmail.com>

* Use SendAsync+Flush instead of SendSync

Signed-off-by: Maxim Zhiburt <zhiburt@gmail.com>

* Add timeout parameter to sendemail command

Signed-off-by: Maxim Zhiburt <zhiburt@gmail.com>

* Fix spelling mistake

Signed-off-by: Maxim Zhiburt <zhiburt@gmail.com>

* Update cmd/admin.go

Co-authored-by: 6543 <6543@obermui.de>

* Connect to a running Gitea instance

* Fix mispelling

* Add copyright comment

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

cmd/mailer.go

@@ -0,0 +1,48 @@
+// 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.
+
+package cmd
+
+import (
+	"fmt"
+	"net/http"
+
+	"code.gitea.io/gitea/modules/private"
+	"github.com/urfave/cli"
+)
+
+func runSendMail(c *cli.Context) error {
+	if err := argsSet(c, "title"); err != nil {
+		return err
+	}
+
+	subject := c.String("title")
+	confirmSkiped := c.Bool("force")
+	body := c.String("content")
+
+	if !confirmSkiped {
+		if len(body) == 0 {
+			fmt.Print("warning: Content is empty")
+		}
+
+		fmt.Print("Proceed with sending email? [Y/n] ")
+		isConfirmed, err := confirm()
+		if err != nil {
+			return err
+		} else if !isConfirmed {
+			fmt.Println("The mail was not sent")
+			return nil
+		}
+	}
+
+	status, message := private.SendEmail(subject, body, nil)
+	if status != http.StatusOK {
+		fmt.Printf("error: %s", message)
+		return nil
+	}
+
+	fmt.Printf("Succseded: %s", message)
+
+	return nil
+}