Add embed-responsive wrapper and classes to media links when using Bootstrap extension

[RudeySH]

Bootstrap provides classes for embeds: https://getbootstrap.com/docs/4.4/utilities/embed/

The Bootstrap extension currently adds img-fluid to media links, because media links are treated as images. Instead, it should add embed-responsive-item. The iframe needs to be surrounded with a div with the following classes: embed-responsive embed-responsive-16by9 (the aspect ratio could be configurable).