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/forms/auth_form.go

@@ -50,6 +50,9 @@ type AuthenticationForm struct {
 	SecurityProtocol              int `binding:"Range(0,2)"`
 	TLS                           bool
 	SkipVerify                    bool
+	HeloHostname                  string
+	DisableHelo                   bool
+	ForceSMTPS                    bool
 	PAMServiceName                string
 	PAMEmailDomain                string
 	Oauth2Provider                string