Convert FalseLazyObject to CheckableLazyObject, use to wrap nonce

[jwhitlock]

To further fix #269, add and test a class CheckableLazyObject, which tests True if the object has been accessed and the value generated, and False if it is still "lazy".

Use this as the wrapper for request.csp_nonce, so that code can test if the nonce is activated before using it:

if (request.csp_nonce):
  print(f"The nonce is set to {request.csp_nonce}")
else:
  print("The nonce has not been set.")

This required a change to the template helpers, to ensure the nonce was read with str(request.csp_nonce). Otherwise, they are treated as falsey.

[robhudson]

Good stuff.