specific thumbnail generation view + manual refresh thumbnail generation, progress bar with error handling and thumbnail deletion

viewer/migrations/0006_file_thumbnail.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.1.2 on 2020-11-01 03:11
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('viewer', '0005_serveddirectory_known_subdirectories'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='file',
+            name='thumbnail',
+            field=models.CharField(default=None, max_length=160, null=True, verbose_name='Thumbnail Filename'),
+        ),
+    ]