Format as data time rejecting valid format

[guilhermekf]

Currently the format_as_date_time filter only accept timezones with 4 digits.
This mean that this date is invalid: 20180703212010+00

The hl7 definition for the CDA time use the TS: PointInTime:
http://hl7.org/cda/stds/core/draft1/StructureDefinition-TS.html

And the PointInTime (TS) definition says that it uses the ISO 8601
https://www.hl7.org/documentcenter/public/wg/inm/datatypes-its-xml20050714.htm (2.32)

ISO 8601 should accept time zone as 2 digits