Abbreviate author/committer in Diffusion directory views

Summary:
The tables are currently kind of wide, and the emails for
non-recognized users seem like the least useful parts, so I put them in
tooltips so they're only visible if you want them. On the other hand, it
means you can only view one at a time, and if you're on mobile you can't
see them at all. Overall, not sure whether this is a good idea.

Test Plan:
Load Diffusion pages with some recognized and some
unrecognized users. Hover over the latter and see that emails appear.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3123