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.

Hello and thank you for wanting to contribute to the MagicMirror²
project

**Please make sure that you have followed these 4 rules before
submitting your Pull Request:**

> 1. Base your pull requests against the `develop` branch.
>
> 2. Include these infos in the description:
>
> - Does the pull request solve a **related** issue?
> - If so, can you reference the issue like this `Fixes
#<issue_number>`?
> - What does the pull request accomplish? Use a list if needed.
> - If it includes major visual changes please add screenshots.
>
> 3. Please run `npm run lint:prettier` before submitting so that
>    style issues are fixed.
>
> 4. Don't forget to add an entry about your changes to
>    the CHANGELOG.md file.

**Note**: Sometimes the development moves very fast. It is highly
recommended that you update your branch of `develop` before creating a
pull request to send us your changes. This makes everyone's lives
easier (including yours) and helps us out on the development team.

Thanks again and have a nice day!

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

modules/default/calendar/calendar.js

@@ -665,7 +665,9 @@ Module.register("calendar", {
 			if (typeof ev.symbol !== "undefined" && ev.symbol !== "") {
 				let needle = new RegExp(ev.keyword, "gi");
 				if (needle.test(event.title)) {
-					symbols[0] = ev.symbol;
+					// Get the default prefix for this class name and add to the custom symbol provided
+					const className = this.getCalendarProperty(event.url, "symbolClassName", this.config.defaultSymbolClassName);
+					symbols[0] = className + ev.symbol;
 					break;
 				}
 			}