[Xon] Email Queuing Enhancements 2.4.2

• Fix "test outbound email" page would queue preventing sending email diagnostics from being outputted

Fix typo in class name breaking email sending

• Fix typo in class name breaking email sending

• Fix uninstaller erroring with XF2.2+
• Remove custom failed queuing logic, use XF native version but with custom next retry policy to be less aggressive about removing failed email.
  • This addon will likely need updating for when XF2.3+ changes email queuing behaviour, but should be a very small update.