Consolidate the page thumbnail and staging folder thumbnail widgets.

Update the staging folder file image API end point to work
in the same way the document page image API works.

mayan/apps/sources/apps.py

@@ -29,7 +29,7 @@ from .links import (
     link_setup_source_edit, link_setup_source_logs, link_staging_file_delete,
     link_upload_version
 )
-from .widgets import staging_file_thumbnail
+from .widgets import StagingFileThumbnailWidget
 
 
 class SourcesApp(MayanAppConfig):
@@ -67,13 +67,12 @@ class SourcesApp(MayanAppConfig):
             func=lambda context: context['object'].get_date_time_created()
         )
 
+        html_widget = StagingFileThumbnailWidget()
         SourceColumn(
             source=StagingFile,
             label=_('Thumbnail'),
-            func=lambda context: staging_file_thumbnail(
-                context['object'],
-                gallery_name='sources:staging_list',
-                title=context['object'].filename, size='100'
+            func=lambda context: html_widget.render(
+                instance=context['object'],
             )
         )