Revert "security: remove our CSRF tokens to improve security and reduce complexity of our threat model (#27780)"

Created by: mrnugget

This reverts commit b7b29868f1e00c8e9057e7ef38ff42a2a7b68db3 because it broke the build pipeline and stopped all deployments.

Example failure: https://buildkite.com/sourcegraph/sourcegraph/builds/117136#dee7e12d-8fa0-49b1-89fa-10c7dc923fbf