feat: add thumbnail field to video xblock fields

cms/templates/widgets/tabs/metadata-edit-tab.html

@@ -1,6 +1,7 @@
 <%namespace name='static' file='../../static_content.html'/>
 <%
   import json
+  import copy
 %>
 
 ## js templates
@@ -13,5 +14,10 @@
         <%static:include path="js/${template_name}.underscore" />
     </script>
 % endfor
-
-<div class="wrapper-comp-settings metadata_edit" id="settings-tab" data-metadata='${json.dumps(editable_metadata_fields) | h}'></div>
+<% metadata_field_copy = copy.copy(editable_metadata_fields) %>
+## Delete 'thumbnail' field (if it exists) so metadata editor view does not attempt to render it.
+% if 'thumbnail' in editable_metadata_fields:
+    ## the new video editor needs access to the 'thumbnail' value, so delete from a copy.
+    <% del metadata_field_copy['thumbnail'] %>
+% endif
+<div class="wrapper-comp-settings metadata_edit" id="settings-tab" data-metadata='${json.dumps(metadata_field_copy) | h}'></div>

xmodule/video_module/video_xfields.py

@@ -212,3 +212,11 @@ class VideoFields:
         scope=Scope.settings,
         default=False
     )
+    # thumbnail is need as a field for the new video editor. The field is hidden in
+    # the legacy modal.
+    thumbnail = String(
+        help=_("Add a specific thumbnail for learners to see before playing the video."),
+        display_name=_("Thumbnail"),
+        scope=Scope.settings,
+        default="",
+    )