MDL-52954 core: Add a document converter to file_storage

This lets us convert between common office formats. E.g. docx -> pdf
html -> pdf, html -> ods.

This commit also updates assignment editpdf plugin to use this converter
on all compatible submission files.

lib/phpunit/bootstrap.php

@@ -185,7 +185,8 @@ $CFG->dboptions = isset($CFG->phpunit_dboptions) ? $CFG->phpunit_dboptions : $CF
 $allowed = array('wwwroot', 'dataroot', 'dirroot', 'admin', 'directorypermissions', 'filepermissions',
                  'dbtype', 'dblibrary', 'dbhost', 'dbname', 'dbuser', 'dbpass', 'prefix', 'dboptions',
                  'proxyhost', 'proxyport', 'proxytype', 'proxyuser', 'proxypassword', 'proxybypass', // keep proxy settings from config.php
-                 'altcacheconfigpath', 'pathtogs', 'pathtodu', 'aspellpath', 'pathtodot'
+                 'altcacheconfigpath', 'pathtogs', 'pathtodu', 'aspellpath', 'pathtodot',
+                 'pathtopandoc'
                 );
 $productioncfg = (array)$CFG;
 $CFG = new stdClass();