Add an explicit "max-width" to PHUIDocumentPro pages to force large tables to scroll

Summary:
Ref T13202. See https://discourse.phabricator-community.org/t/phriction-page-controls-lost-after-creating-very-wide-table/1961.

If you put a very wide table in the markup for a new-layout Phriction page, it can push the actions element off screen to the right.

Tables already get a scrollbar if encouraged strongly enough; add a max-width to encourage them.

Test Plan:

• Viewed pages with a large wrappable and non-wrappable content on mobile, tablet, and desktop.

{F5915976}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13202

Differential Revision: https://secure.phabricator.com/D19723