#1171: Use the ical.js implementation for foldline

According to [RFC5545](https://www.rfc-editor.org/rfc/rfc5545#section-3.1):

```
  Note: It is possible for very simple implementations to generate
  improperly folded lines in the middle of a UTF-8 multi-octet
  sequence.  For this reason, implementations need to unfold lines
  in such a way to properly restore the original sequence.
```

It actually happens when an emoji takes the 75th and 76th bit.

www/calendar/export.js

@@ -96,6 +96,41 @@ define([
                 return rrule;
             };
 
+            // XXX Below function: from ical.js, should be imported instead.
+            // XXX However, if it's done, it would be better to refactor the
+            // XXX export module to directly use ical.js for exporting ICS.
+            var foldline = function(aLine) {
+                var foldLength = 75;
+                var newLineChar = '\r\n';
+                let result = "";
+                let line = aLine || "", pos = 0, line_length = 0;
+                //pos counts position in line for the UTF-16 presentation
+                //line_length counts the bytes for the UTF-8 presentation
+                while (line.length) {
+                    let cp = line.codePointAt(pos);
+                    if (cp < 128) {
+                        ++line_length;
+                    }
+                    else if (cp < 2048) {
+                        line_length += 2;//needs 2 UTF-8 bytes
+                    }
+                    else if (cp < 65536) {
+                        line_length += 3;
+                    }
+                    else {
+                        line_length += 4; //cp is less than 1114112
+                    }
+                    if (line_length < foldLength + 1) {
+                        pos += cp > 65535 ? 2 : 1;
+                    }
+                    else {
+                        result += newLineChar + " " + line.slice(0, Math.max(0, pos));
+                        line = line.slice(Math.max(0, pos));
+                        pos = line_length = 0;
+                    }
+                }
+                return result.slice(newLineChar.length + 1);
+            };
 
             var addEvent = function (arr, data, recId) {
                 var uid = data.id;
@@ -105,11 +140,12 @@ define([
                 var rrule = getRRule(data);
 
                 var formatDescription = function(str) {
-                    var result = str.replace(/\n/g, ["\\n"]);
+                    var componentName = 'DESCRIPTION:';
+                    var result = componentName + str.replace(/\n/g, ["\\n"]);
                     // XXX Should use ical.js helper foldline instead ↓
                     // XXX https://kewisch.github.io/ical.js/api/ICAL.module_helpers.html#.foldline
                     // In RFC5545: https://www.rfc-editor.org/rfc/rfc5545#section-3.1
-                    result = result.replace(/([^\n]{1,74})/g, '$1\n ');
+                    result = foldline(result);
                     return result;
                 };
 
@@ -123,7 +159,7 @@ define([
                     rrule,
                     'SUMMARY:'+ data.title,
                     'LOCATION:'+ data.location,
-                    'DESCRIPTION:' + formatDescription(data.body),
+                    formatDescription(data.body),
                 ].filter(Boolean));
 
                 if (Array.isArray(data.reminders)) {