R1:42cf7f6faa10

Make the current session key a component of the CSRF token

Summary: Fixes T5510. This purely reduces false positives from HackerOne: we currently rotate CSRF tokens, but do not bind them explicitly to specific sessions. Doing so has no real security benefit and may make some session rotation changes more difficult down the line, but researchers routinely report it. Just conform to expectations since the expected behavior isn't bad and this is less work for us than dealing with false positives.

Test Plan…