MDL-83543 core: Move download complete status handling to Excel library

When a form is validated, all validation buttons are automatically
locked to prevent them from being sent twice by mistake. When the form
returns a file instead of redirecting to a new HTML page, a mechanism
(\core_form\util::form_download_complete()) must be called to
reactivate the form's validation buttons.

This mechanism is called for CSV files (file lib/csvlib.class.php), but
not when downloading xls files. The mechanism is only called when
downloading grades in Excel format (file
grade/export/xls/grade_export_xls.php).

The patch proposes to call this mechanism globally for all Excel file
downloads (file lib/excellib.class.php), and not just for grade
downloads (file grade/export/xls/grade_export_xls.php).

public/grade/export/xls/grade_export_xls.php

@@ -45,9 +45,6 @@ class grade_export_xls extends grade_export {
 
         $strgrades = get_string('grades');
 
-        // If this file was requested from a form, then mark download as complete (before sending headers).
-        \core_form\util::form_download_complete();
-
         // Calculate file name
         $shortname = format_string($this->course->shortname, true, array('context' => context_course::instance($this->course->id)));
         $downloadfilename = clean_filename("$shortname $strgrades.xls");

public/lib/excellib.class.php

@@ -110,6 +110,9 @@ class MoodleExcelWorkbook {
     public function close() {
         global $CFG;
 
+        // If this file was requested from a form, then mark download as complete.
+        \core_form\util::form_download_complete();
+
         foreach ($this->objspreadsheet->getAllSheets() as $sheet) {
             $sheet->setSelectedCells('A1');
         }