Fix long URLs on mobile chatlog

Summary: Really long words or URLs from IRC break mobile layout, so this breaks the word if it hits the edge of the screen.

Test Plan: Duderpooooooooooooooooooooooooooooooooooooooooo

Reviewers: epriestley, btrahan

Reviewed By: btrahan

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5703