Encode "<" and ">" in JSON/Ajax responses to prevent content-sniffing attacks

Summary:
Some browsers will still sniff content types even with "Content-Type" and
"X-Content-Type-Options: nosniff". Encode "<" and ">" to prevent them from
sniffing the content as HTML.

See T865.

Also unified some of the code on this pathway.

Test Plan: Verified Opera no longer sniffs the Conduit response into HTML for
the test case in T865. Unit tests pass.

Reviewers: cbg, btrahan

Reviewed By: cbg

CC: aran, epriestley

Maniphest Tasks: T139, T865

Differential Revision: https://secure.phabricator.com/D1606