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/constants.py

@@ -37,6 +37,7 @@ else:
 STATIC_DIR          = os.path.join(BASE_DIR, 'cps', 'static')
 TEMPLATES_DIR       = os.path.join(BASE_DIR, 'cps', 'templates')
 TRANSLATIONS_DIR    = os.path.join(BASE_DIR, 'cps', 'translations')
+CACHE_DIR           = os.path.join(BASE_DIR, 'cps', 'cache')
 
 if HOME_CONFIG:
     home_dir = os.path.join(os.path.expanduser("~"),".calibre-web")