[routes] adapt CookielessConverter to new werkzeug router

The new router does not perform correctly when using regex groups. Moreover,
it is now required to explicitely tell it the converter can match the forward /
using ``part_isolating``.

inginious/frontend/flask/mapping.py

@@ -51,7 +51,8 @@
 
 class CookielessConverter(BaseConverter):
     # Parse the cookieless sessionid at the beginning of the url
-    regex = r"((@)([a-f0-9A-F_]*)(@/))?"
+    regex = "@[a-f0-9A-F_]*@/|"
+    part_isolating = False
 
     def to_python(self, value):
         return value[1:-2]