Harden default CSP
Harden default CSP

rehabstudio/rehabstudio-gae-scaffold,google/gae-secure-scaffold-python,google/gae-secure-scaffold-python,google/gae-secure-scaffold-python,google/gae-secure-scaffold-python3,rehabstudio/rehabstudio-gae-scaffold,rehabstudio/rehabstudio-gae-scaffold