HTML entities in iCalendar export

Overview

iCalendar (iCal) export does not decode html entities.

Reproduction steps

1. Create an event or activity with a html description containing special characters like German umlauts or non breaking spaces.
2. Export this entity as ical and open it in a mail client like Microsoft Outlook.
3. Note that text contains html entities like ü or &npsp;

Current behaviour

Example excerpt from iCalendar export:

DESCRIPTION:Ein Thema der Fachveranstaltung ist die Digitalisi
 erung im Bereich der Agrarwirtschaft\, unter ander
 em&nbsp\;wie die Messung und Speicherung von Daten
 &nbsp\;landwirtschaftlich genutzt werden k&ouml\;n
 nen.\n \n Die Teilnehmer k&ouml\;nnen aschlie&szli
 g\;end&nbsp\;innerhalb von Workshops zum Thema Bio
 gasanlage und Energieeffizienz ihr Wissen erweiter
 n.

Expected behaviour

Decode all html entities for safe iCalendar use.

Environment information

• CiviCRM: Master
• PHP: 7.2
• CMS: Drupal 7.30

Comments

This issue was already reported in https://issues.civicrm.org/jira/browse/CRM-13645 Based on this I've created a patch, which fixes the issue for me: ical_htmlentitydecode.patch

Note that html in iCalendar is fine if the "Alternate Text Representation" points to a "text/html" content portion (see https://tools.ietf.org/html/rfc5545#section-3.2.1 for details).