Use absolute widths for Maniphest task columns instead of "min-width" plus "width: ...%"

Summary:
There's an occasional display glitch with the current CSS
(http://cl.ly/2B0Q2l3T0N2n3M0k092A) that we think this will fix.

Seems least-bad in light of this:
https://secure.phabricator.com/file/data/2f5wamew66aggnlqw7oo/PHID-FILE-aikqvnrmw525cn2wfzb2/widenarrow.png

Test Plan: Looked at Maniphest in a couple of browsers at different screen
widths.

Reviewers: paularmstrong, btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1669