Added handling for missing flask-wtf dependency

Added CSRF protection (via flask-wtf)
Moved upload function to js file
Fixed error page in case of csrf failure

setup.cfg

@@ -18,9 +18,11 @@ classifiers =
     Development Status :: 5 - Production/Stable
     License :: OSI Approved :: GNU Affero General Public License v3
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
+    Programming Language :: Python :: 3.10
     Operating System :: OS Independent
 keywords =
   calibre
@@ -49,6 +51,7 @@ install_requires =
     Wand>=0.4.4,<0.7.0
     unidecode>=0.04.19,<1.3.0
     lxml>=3.8.0,<4.7.0
+    flask-wtf>=0.15.0,<0.16.0
 
 [options.extras_require]
 gdrive =