[New metadata element]: issue_tracker

[matentzn]

Element id (e.g. creator_id, mapping_tool_version):
(Must be lower case and contain only letters and underscores.)

issue_tracker

Value data type (e.g. URI, URL, text, xsd:boolean):

URL

Description
(Provide a human-readable description that clarifies the intended use of the metadata element.)

The issue tracker that can be used to report bugs and request changes for this mapping set.

EDIT:

I propose to add issue_tracker and issue_tracker_item separately now; see #259

[matentzn]

I propose to add issue_tracker and issue_tracker_item separately now; see #259