Improve rendering for Maniphest custom fields

Summary: We render these in a realtively unreadable way right now; allow customization and provide reasonable defaults.

Test Plan: Looked at some tasks with custom fields on them.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T930

Differential Revision: https://secure.phabricator.com/D1790