MDL-60319 Google converter: Add html to convertible types

Add html files to the file types that Google can convert. This allows
the Google Drive converter to convert online submissions to pdf. This
fixes an issue where if online submissions is selected and the Google
Drive converter is used all conversions fail and produce a single
blank pdf. Credit to Peter Svec for the idea for this fix.

files/converter/googledrive/classes/converter.php

@@ -47,7 +47,8 @@ class converter implements \core_files\converter_interface {
         'xls' => 'application/vnd.google-apps.spreadsheet',
         'xlsx' => 'application/vnd.google-apps.spreadsheet',
         'ppt' => 'application/vnd.google-apps.presentation',
-        'pptx' => 'application/vnd.google-apps.presentation'
+        'pptx' => 'application/vnd.google-apps.presentation',
+        'html' => 'application/vnd.google-apps.document'
     ];
 
     /** @var array $export List of supported export file formats */
@@ -265,6 +266,6 @@ public static function supports($from, $to) {
      * @return  string
      */
     public function get_supported_conversions() {
-        return implode(', ', ['rtf', 'doc', 'xls', 'docx', 'xlsx', 'ppt', 'pptx', 'pdf']);
+        return implode(', ', ['rtf', 'doc', 'xls', 'docx', 'xlsx', 'ppt', 'pptx', 'pdf', 'html']);
     }
 }