Automatically parse {__html: "<markup>"} into calls to JX.$H
Automatically parse {__html: "<markup>"} into calls to JX.$H

Summary:
If a server endpoint returns escaped markup, the client must call
JX.$H to render it properly. This is dangerous if ever a server makes a mistake
or a mismatch occurs between client code expecting escaped markup and a server
endpoint returning raw strings.

To replace this use case for $H, this builds a protocol into JX.Request to
understand JSON objects augmented with wrapped strings. Strings that are
wrapped in objects like {__html: "<markup">} are automatically parsed
by JX.Request into $H in one place. With this in place and appropriate
abstractions on the server, client code should never have to call $H directly.

Marshall envisions potentially extending this parsing to include something like
__e
to call JX.$ automatically, and perhaps others.

I considered checking that the object was *exactly* {__html: something}, i.e.
had no other keys, but decided that if someone was inadvertently sending down
__html
in non-raw requests, they're confused, and selectively applying the rule would
only increase the chance of making it a heisenbug.

Test Plan: Load diffs in Phabricator, deliberately break it, see diff loads
fail, unbreak it.

Reviewers: mroch, epriestley

Reviewed By: epriestley

CC: tomo, aran, epriestley

Differential Revision: 1217

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