Change the alert banner summary to be semicolon separated with alert count as part of summary

Andy Broomfieldrequested to merge
issue/localgov_alert_banner_collapsible-3570313:3570313-change-the-format into 1.x
  • Use the 3 alerts: banner 1; banner 2; and 1 more style
  • Move the alert count from the template into the summary
  • Adds event subscriber so modules can generate the summary
  • Adds max_before_summary to the render array to help themes

Closes #3570313

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