Fix #1930964 [[Enhancement - Server] Show a warning when not having applied changes after editing metadata](https://bugs.launchpad.net/calibre/+bug/1930964)

src/pyj/book_list/edit_metadata.pyj

@@ -2,10 +2,9 @@
 # License: GPL v3 Copyright: 2018, Kovid Goyal <kovid at kovidgoyal.net>
 from __python__ import bound_methods, hash_literals
 
-import traceback
 from elementmaker import E
-from gettext import gettext as _
 
+import traceback
 from ajax import ajax_send
 from book_list.book_details import (
     add_stars_to, basic_table_rules, fetch_metadata, field_sorter, no_book,
@@ -29,7 +28,8 @@ from dom import add_extra_css, build_rule, clear, svgicon
 from file_uploads import (
     update_status_widget, upload_files_widget, upload_status_widget
 )
-from modals import error_dialog
+from gettext import gettext as _
+from modals import error_dialog, question_dialog
 from session import get_interface_data
 from utils import (
     conditional_timeout, fmt_sidx, parse_url_params, safe_set_inner_html
@@ -1054,6 +1054,14 @@ def on_close(container_id):
                 {'type': 'edit_metadata_closed', 'from_read_book': cq.from_read_book},
                 document.location.protocol + '//' + document.location.host
             )
+        else:
+            if has_changes:
+                question_dialog(_('Are you sure'), _(
+                    'Any changes you have made will be discarded.'
+                    ' Do you wish to discard all changes?'), def (yes):
+                        if yes:
+                            back()
+                )
             else:
                 back()