This change adds support for retrieving an antiforgery CSRF token via a
configurable header in addition to the form field. This helps with doing
ajax requests in a 1st-party SPA when using cookie auth, and is similar to
functionality provided by a bunch of different frameworks.
In this change there's also a bunch of churn due to avoiding the term
'form' in favor of 'request' and 'session' in favor of 'cookie'. Where
code and error message now mention 'form' they specifically mean
form-encoded content.
Ryan Nowak
Pranav K
Hao Kung