Setting WTF_CSRF_TIME_LIMIT to "None" is confusing

[malthe]

In #258, it is made "clear" that WTF_CSRF_TIME_LIMIT can be set to None. But this isn't entirely clear because what is None in the context of an environment variable?

Is it the absence of a value (presumably not, because then we get the default of 3600 seconds) - ?

Is it an empty string? I think a better value for the behavior of "limited by user session" would be 0. This works better with configuration by environment variable.