Added thumbnails

Merge remote-tracking branch 'cover/thumbnails' into development

# Conflicts:
#	cps/admin.py
#	cps/templates/layout.html
#	cps/ub.py
#	cps/web.py
Update join for sqlalchemy 1.4

cps/schedule.py

@@ -0,0 +1,36 @@
+# -*- coding: utf-8 -*-
+
+#   This file is part of the Calibre-Web (https://github.com/janeczku/calibre-web)
+#     Copyright (C) 2020 mmonkey
+#
+#   This program is free software: you can redistribute it and/or modify
+#   it under the terms of the GNU General Public License as published by
+#   the Free Software Foundation, either version 3 of the License, or
+#   (at your option) any later version.
+#
+#   This program is distributed in the hope that it will be useful,
+#   but WITHOUT ANY WARRANTY; without even the implied warranty of
+#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#   GNU General Public License for more details.
+#
+#   You should have received a copy of the GNU General Public License
+#   along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+from __future__ import division, print_function, unicode_literals
+
+from .services.background_scheduler import BackgroundScheduler
+from .tasks.database import TaskReconnectDatabase
+from .tasks.thumbnail import TaskSyncCoverThumbnailCache, TaskGenerateCoverThumbnails
+
+
+def register_jobs():
+    scheduler = BackgroundScheduler()
+
+    # Generate 100 book cover thumbnails every 5 minutes
+    scheduler.add_task(user=None, task=lambda: TaskGenerateCoverThumbnails(limit=100), trigger='cron', minute='*/5')
+
+    # Cleanup book cover cache every 6 hours
+    scheduler.add_task(user=None, task=lambda: TaskSyncCoverThumbnailCache(), trigger='cron', minute='15', hour='*/6')
+
+    # Reconnect metadata.db every 4 hours
+    scheduler.add_task(user=None, task=lambda: TaskReconnectDatabase(), trigger='cron', minute='5', hour='*/4')