Reapply fix by Florian Pritz: change DATETIME to DATE-TIME in vcards

The rfc [1] only mentions "DATE-TIME" and after this change events
created in the web ui can be imported by lightning. Before it threw a
syntax error.

[1]: https://tools.ietf.org/html/rfc5545



Signed-off-by: Florian Pritz <bluewind@xinu.at>

From commit 71706282