Release 21-07-20

• [BUGFIX] Use current user if no owner was specified
• [BUGFIX] Keep extension key throws exception
• [BUGFIX] Zipped extension includes empty files
• [TASK] Move core version check to LTSVersionService
• [TASK] Merge tx_ter_extensiondetails into tx_ter_extensions
• [TASK] Drop unused ext_emconf v1 fields from TER v2