Merged Datasets

• Introducing merged datasets (#1057)
• Licence copyright update (4913c6f)
• Support switching to a different user for Oauth2 Apps (#1062)
• Task to clean up pending and failed exports (#1068)
• Keeping track of received attachments per submission (#1049)
• New /data endpoint filters: date_created, date_modified, status,submitted_by, version (#1048)
• Bug fixes (#954, #983, #1069, #1072, #1077, #1087, #1088, #1093, #1107)