Fix header display bug on forked pastes.

Summary:
You can only call setHeader() on a Panel once. Otherwise the last sticks. Move the "forks of this paste" stuff to its own panel (only shown if there are, indeed, forks), and make the columns look nicer.

Test Plan:
Viewed previously forked pastes, forked a paste and looked at the original, and looked at a non-forked paste. All looked sane.

Reviewers:
epriestley

CC:

Differential Revision: 700