Changed behavior of "code block" button to use triple tick syntax ("```") instead of whitespacing.

Summary:
This is important when dealing in situations where there's ambiguity between code and lists, like with Objective-C code:

  - (void) foobar {}

will get your a list instead of the desired code block

Test Plan: Tested locally

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9379