Improve SMTP authentication and Fix user creation bugs (#16612)

* Improve SMTP authentication, Fix user creation bugs and add LDAP cert/key options

This PR has two parts:

Improvements for SMTP authentication:

* Default to use SMTPS if port is 465, and allow setting of force SMTPS.
* Always use STARTTLS if available
* Provide CRAM-MD5 mechanism
* Add options for HELO hostname disabling
* Add options for providing certificates and keys
* Handle application specific password response as a failed user login
instead of as a 500.

Close #16104

Fix creation of new users:

* A bug was introduced when allowing users to change usernames which
prevents the creation of external users.
* The LoginSource refactor also broke this page.

Close #16104

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

services/auth/source/smtp/source.go

@@ -22,8 +22,10 @@ type Source struct {
 	Host           string
 	Port           int
 	AllowedDomains string `xorm:"TEXT"`
-	TLS            bool
+	ForceSMTPS     bool
 	SkipVerify     bool
+	HeloHostname   string
+	DisableHelo    bool
 
 	// reference to the loginSource
 	loginSource *models.LoginSource
@@ -51,7 +53,7 @@ func (source *Source) HasTLS() bool {
 
 // UseTLS returns if TLS is set
 func (source *Source) UseTLS() bool {
-	return source.TLS
+	return source.ForceSMTPS || source.Port == 465
 }
 
 // SetLoginSource sets the related LoginSource