Add-ons Email Queuing Enhancements 2.3.0

• Require php 7.2+
• Require XenForo 2.2+
• Require StandardLib v1.18.0+
• 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.