Align table cell content in Diffusion.

Summary: Most cells in Diffusion were aligning text to the top, and the sigil height is fixed. This lines all up on center.

Test Plan: Tested diffusion with new cell alignment, clicked around to other tables. Looks good in Chrome.

Reviewers: epriestley, asherkin, btrahan

Reviewed By: btrahan

CC: aran, Korvin

Maniphest Tasks: T2189

Differential Revision: https://secure.phabricator.com/D4210