Add-ons Alert Improvements by Xon 2.13.1

• Fix regression in v2.9.0 where empty content summary alerts was being generated, and incorrectly preferring a per-user summary instead
• Fix that editing alert preferences when summarization is disabled, would disable some summarization options

• Require Standard Library by Xon v1.18.0+
• Fix alert summarizing only triggered on +25 unread alerts instead of the user-defined threshold
• Fix summary alerts in the alert pop-up would be pushed under the just read alerts as if it had been read a while ago
• Adjust "Alert summarize limit" option description text to better describe what a limit of 0 does (ie no limit)

• Fix "Alert summarization" option being disabled still showed user's preference options to configure it