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/configs/modules/calendar/custom.js

@@ -11,9 +11,10 @@ let config = {
 			module: "calendar",
 			position: "bottom_bar",
 			config: {
+				customEvents: [{ keyword: "CustomEvent", symbol: "dice" }],
 				calendars: [
 					{
-						maximumEntries: 4,
+						maximumEntries: 5,
 						maximumNumberOfDays: 10000,
 						symbol: "birthday-cake",
 						fullDaySymbol: "calendar-day",