Fix some Paste scroll issues

Summary:

• If a Paste has very long lines, we show far too much text in the summary, because we wrap the lines. Instead, overflow them.
• If a Paste has very long unbroken lines (MMMMM...), they extend past the page. Instead, add a scrollable container.

Test Plan:
{F27868}

{F27869}

{F27870}

Reviewers: btrahan, codeblock, chad

Reviewed By: codeblock

CC: aran

Differential Revision: https://secure.phabricator.com/D4211