export icalendar file on_change hook

[roquegv]

issue #50

This is how it works:

This is the generated file (the original file was compressed into .zip because GitHub does not allow .ics files):
Urlaubskalender499864.zip

Observation:

• The export function will be triggered when there is a change in a Leave Application document, particularly when it is in a "Approved" status.
• The File is attached to the last Leave Application in "Approved" status, which triggered the function call.
• The generated filename is always unique, because the content's hash is suffixed to the original filename. This behavior cannot be changed using the standard frappe.utils.file_manager.save_file function. If we always need the same filename, then we should follow a different function and this may take a while to make it working appropriately.

[marcus-wiegand]

ical Data:

BEGIN:VCALENDAR
BEGIN:VEVENT
SUMMARY:HR-EMP-00001 - Sick Leave
DTSTART;VALUE=DATE:20230804
DTEND;VALUE=DATE:20230804
DESCRIPTION:cold
END:VEVENT
BEGIN:VEVENT
SUMMARY:HR-EMP-00001 - Sick Leave
DTSTART;VALUE=DATE:20230802
DTEND;VALUE=DATE:20230802
DESCRIPTION:cold
END:VEVENT
BEGIN:VEVENT
SUMMARY:HR-EMP-00001 - Sick Leave
DTSTART;VALUE=DATE:20230801
DTEND;VALUE=DATE:20230801
DESCRIPTION:flu
END:VEVENT
BEGIN:VEVENT
SUMMARY:HR-EMP-00001 - Sick Leave
DTSTART;VALUE=DATE:20230803
DTEND;VALUE=DATE:20230803
DESCRIPTION:fever
END:VEVENT
END:VCALENDAR