Sendmail should create a process on the gitea system and have a default timeout (#11256)

* Make sure that sendmail processes register with the process manager
* Provide a timeout for these (initially of 5 minutes)
* Add configurable value and tie in to documentation
* Tie in to the admin config page.

Signed-off-by: Andrew Thornton <art27@cantab.net>

options/locale/locale_en-US.ini

@@ -2119,6 +2119,7 @@ config.mailer_user = User
 config.mailer_use_sendmail = Use Sendmail
 config.mailer_sendmail_path = Sendmail Path
 config.mailer_sendmail_args = Extra Arguments to Sendmail
+config.mailer_sendmail_timeout = Sendmail Timeout
 config.send_test_mail = Send Testing Email
 config.test_mail_failed = Failed to send a testing email to '%s': %v
 config.test_mail_sent = A testing email has been sent to '%s'.