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

vulnerabilities or weird edge cases that we can fix. But don't push to Facebook
for a few days unless you're prepared to deal with this.

• **>>> MEGADERP: All Conduit write APIs are currently vulnerable to CSRF!

<<<**

Test Plan:

• Ran some scripts that perform writes (scripts/search indexers), no issues.
• Performed normal CSRF submits.
• Added writes to an un-CSRF'd page, got an exception.
• Executed conduit methods.
• Did login/logout (this works because the logged-out user validates the

logged-out csrf "token").

• Did OAuth login.
• Did OAuth registration.

Reviewers: pedram, andrewjcg, erling, jungejason, tuomaspelkonen, aran,
codeblock
Commenters: pedram
CC: aran, epriestley, pedram
Differential Revision: 777