iCalendar export: default rely on value types

[dilyanpalauzov]

Per https://tools.ietf.org/html/rfc5545#section-3.8.2.4 the default value type for DTSTART, DTEND and DTSTAMP is DATE-TIME.

Calling https://indico.geant.org/export/categ/0.ics returns:

DTSTART;VALUE=DATE-TIME:20190616T050000Z
DTEND;VALUE=DATE-TIME:20190620T153000Z
DTSTAMP;VALUE=DATE-TIME:20210217T082500Z

In order to reduce the amount of transferred data remove the ;VALUE=DATE-TIME string, as it is implicit.

[ThiefMaster]

We use the icalendar library; I don't think it has an option for this. Maybe you want to open an issue on their issue tracker instead, or even send a PR to improve this? :)

[dilyanpalauzov]

Moved do collective/icalendar#318.