R1:39b4d20ce528

Create AphrontWriteGuard, a backup mechanism for CSRF validation

Summary:
Provide a catchall mechanism to find unprotected writes.

- Depends on D758.
- Similar to WriteOnHTTPGet stuff from Facebook's stack.
- Since we have a small number of storage mechanisms and highly structured
read/write pathways, we can explicitly answer the question "is this page
performing a write?".
- Never allow writes without CSRF checks.
- This will probably break some things. That's fine: they're CSRF…