Copy/adapt latest CSRF filter improvements

[ThrawnCA]

There have been significant improvements to https://github.com/qld-gov-au/ckanext-csrf-filter since its code was adapted for this extension. It could be beneficial to copy these changes.

Improvements include:

• Tokens use HMAC signatures with a server secret, to ensure that they cannot be forged.
• Token rotation is more robust, using a timestamp built into the token, and expired tokens are rejected.
• Added support for intercepting login form to prevent Login CSRF.
• Code has been refactored for both Flask and Pylons compatibility, as well as handling both Python 2 and 3. Flask support uses official Flask intercept points and no longer requires monkey-patching core functions.