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