Update page CSS modifier classes for better page content management.

Review Request #10738 — Created Oct. 2, 2019 and submitted — Latest diff uploaded

Review Board

This introduces three new CSS modifier classes for pages:
-is-content-flush, -is-content-flush-on-mobile, and
-is-content-flush-on-desktop. When applied to a page's <body>, these
will remove the margins/padding around the content, making it flush
against the UI shell and browser. It's primarily useful for mobile,
where we have limited screen space and want to remove inner containers
(like the box around a dashboard's contents).

The existing -has-full-page-content option has been renamed to
-is-content-full-page for consistency.

All modifiers are now documented.

Tested that the dashboard still works correctly.

Made use of the new classes in some upcoming changes.