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>

routers/private/internal.go

@@ -47,5 +47,6 @@ func RegisterRoutes(m *macaron.Macaron) {
 		m.Post("/manager/release-and-reopen-logging", ReleaseReopenLogging)
 		m.Post("/manager/add-logger", bind(private.LoggerOptions{}), AddLogger)
 		m.Post("/manager/remove-logger/:group/:name", RemoveLogger)
+		m.Post("/mail/send", SendEmail)
 	}, CheckInternalToken)
 }