Fix calendar.js missing default symbol prefix (#3007)

Symbols provided in customEvents don't get the "fas fa-fw fa-" prefix,
which according to the docs they should. This fixes that.

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Co-authored-by: veeck <michael@veeck.de>

tests/mocks/calendar_test_icons.ics

@@ -53,4 +53,11 @@ RRULE:FREQ=YEARLY;BYMONTH=10;BYMONTHDAY=1
 DTEND;VALUE=DATE:20301002
 SUMMARY:TestEventRepeatDay
 END:VEVENT
-END:VCALENDAR
+BEGIN:VEVENT
+DTSTAMP:20200721T094531Z
+UID:20200719T094531Z-167389794@marudot.com
+DTSTART;TZID=Europe/Berlin:20301112T120000
+DTEND;TZID=Europe/Berlin:20301112T130000
+SUMMARY:TestEventCustomEventIcon
+END:VEVENT
+END:VCALENDAR