Add `Date signed up` field on export lists

[golenkovm]

Need to have date booked shown on attendance lists or reports for single sessions, not only on whole activity export attendance report.

Implementation approach:

• Add additional header column title here after this line $worksheet->write_string($row, $column++, "Date signed up", ['bold' => 1, 'border' => 1]);
• add the following as the last column here after this line $worksheet->write_date($row, $column++, $attendee->timecreated, $format);