Fix subscriptions new video count when there are deleted videos

It would be 30 since the old method looked to see where the latest
video in the database is in the new batch of videos. New method
finds the first video in the new batch which is in the database.

youtube/subscriptions.py

@@ -531,22 +531,19 @@ def find_element(base, tag_name):
         with connection as cursor:
 
             # calculate how many new videos there are
-            row = cursor.execute('''SELECT video_id
-                                    FROM videos
-                                    INNER JOIN subscribed_channels ON videos.sql_channel_id = subscribed_channels.id
-                                    WHERE yt_channel_id=?
-                                    ORDER BY time_published DESC
-                                    LIMIT 1''', [channel_id]).fetchone()
-            if row is None:
-                number_of_new_videos = len(videos)
-            else:
-                latest_video_id = row[0]
-                index = 0
-                for video in videos:
-                    if video['id'] == latest_video_id:
-                        break
-                    index += 1
-                number_of_new_videos = index
+            existing_vids = set(row[0] for row in cursor.execute(
+                '''SELECT video_id
+                   FROM videos
+                   INNER JOIN subscribed_channels
+                       ON videos.sql_channel_id = subscribed_channels.id
+                   WHERE yt_channel_id=?
+                   ORDER BY time_published DESC
+                   LIMIT 30''', [channel_id]).fetchall())
+            number_of_new_videos = 0
+            for video in videos:
+                if video['id'] in existing_vids:
+                    break
+                number_of_new_videos += 1
 
             is_first_check = cursor.execute('''SELECT time_last_checked FROM subscribed_channels WHERE yt_channel_id=?''', [channel_id]).fetchone()[0] in (None, 0)
             time_videos_retrieved = int(time.time())