Fix youtube and wordpress video breaking container

Summary: Youtube videos embedded within an iframe will fill the width of its container as a result of this change. For videos that don't fill the width of the page initially, they will after the aggregator pulls in the content and the styling is applied.

Reviewers: jriddell, bcooksley

Reviewed By: jriddell

Differential Revision: https://phabricator.kde.org/D12796