Mail module should add an option to explicitly decide using tls or not.

[simonwu-os]

Dear All,
in the module gitea/modules/mailer/mailer.go, there is a fragment to decide it using tls or not.
isSecureConn := false
// Start TLS directly if the port ends with 465 (SMTPS protocol)
if strings.HasSuffix(port, "465") {
conn = tls.Client(conn, tlsconfig)
isSecureConn = true
}
It says that the port should suffix with '465'. we have a problem it, because we use a smtp which using the port 946 for tls connect. So I suggest add an option in the mail config to support tls connect explicitly.
Thanks very much.