Remove width restrictions for remarup code blocks

Summary:
Currently, code blocks inside Phabricator remarkup are constrained to ~80col. There's no technical or design reason for this -- it looks and works fine without the restriction. Join the new world of fluid widths.

(Note that inline comments can get a bit whacky in some cases with large code blocks, but we have no restrictions there currently, so this doesn't break anything.)

Test Plan: {F26814}

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2005

Differential Revision: https://secure.phabricator.com/D4135