Fix exercise date parsing for non-group submissions

ILIAS apparently changes the order of the fields as it sees fit, so we
now try to parse *every* column, starting at from the right, as a date.
The first column that parses successfully is then used.

CHANGELOG.md

@@ -22,6 +22,9 @@ ambiguous situations.
 
 ## Unreleased
 
+### Fixed
+- Date parsing now also works correctly in non-group exercises
+
 ## 3.0.0 - 2021-05-31
 
 ### Added