Properly update event ics export cache and fix export desc html

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/service/export/icalendar.ex

@@ -29,7 +29,7 @@ defmodule Mobilizon.Service.Export.ICalendar do
       dtstart: event.begins_on,
       dtstamp: event.publish_at || DateTime.utc_now(),
       dtend: event.ends_on,
-      description: event.description,
+      description: HtmlSanitizeEx.strip_tags(event.description),
       uid: event.uuid,
       categories: event.tags |> Enum.map(& &1.slug)
     }