Improve email test page

[brendanheywood]

• [ ] If you send an email and it bounces this should be shown on admin/testoutgoingmailconf.php with a error

This SHOULD be over the bounce threshold and so email_to_user will return false with a debugging message, but it isn't somehow

This might be a tracker, just keeping visibility of it here

[brendanheywood]

I think the most correct action here is to add a new form field to core:

• send the email as a moodle user if one can be found (this will honour bounces)
• send the email as a generic system user (this will not)

The first will be the default. This logic would apply to both the To user and the From user