* 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>
