R1:6f9683dc904a

Add paste samples to list view.

Summary:
This adds a "body" field to `PhabricatorObjectItemView` which lets you optionally
add more information to the list view. It then uses this new field to show
samples of pastes in the paste list view.

Test Plan:
{F27147}

{F27148}

Reviewers: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4188