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/fs.py

@@ -0,0 +1,61 @@
+# -*- 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 .constants import CACHE_DIR
+from os import listdir, makedirs, remove
+from os.path import isdir, isfile, join
+from shutil import rmtree
+
+CACHE_TYPE_THUMBNAILS = 'thumbnails'
+
+
+class FileSystem:
+    _instance = None
+    _cache_dir = CACHE_DIR
+
+    def __new__(cls):
+        if cls._instance is None:
+            cls._instance = super(FileSystem, cls).__new__(cls)
+        return cls._instance
+
+    def get_cache_dir(self, cache_type=None):
+        if not isdir(self._cache_dir):
+            makedirs(self._cache_dir)
+
+        if cache_type and not isdir(join(self._cache_dir, cache_type)):
+            makedirs(join(self._cache_dir, cache_type))
+
+        return join(self._cache_dir, cache_type) if cache_type else self._cache_dir
+
+    def get_cache_file_path(self, filename, cache_type=None):
+        return join(self.get_cache_dir(cache_type), filename) if filename else None
+
+    def list_cache_files(self, cache_type=None):
+        path = self.get_cache_dir(cache_type)
+        return [file for file in listdir(path) if isfile(join(path, file))]
+
+    def delete_cache_dir(self, cache_type=None):
+        if not cache_type and isdir(self._cache_dir):
+            rmtree(self._cache_dir)
+        if cache_type and isdir(join(self._cache_dir, cache_type)):
+            rmtree(join(self._cache_dir, cache_type))
+
+    def delete_cache_file(self, filename, cache_type=None):
+        if isfile(join(self.get_cache_dir(cache_type), filename)):
+            remove(join(self.get_cache_dir(cache_type), filename))