Allow users to set whether or not textareas are monospaced.

Summary:
Some users like monospaced textareas and others don't.
This introduces an option to set this as a user preference.

Test Plan: Enabled and saw monospaced textareas, disabled and saw non-monospaced textareas.

Reviewers: epriestley

CC: aran, Korvin

Maniphest Tasks: T2114

Differential Revision: https://secure.phabricator.com/D4037