Address lack of viewmedia and viewblock events on blog homepage and topic pages

[justinedelson]

While evaluating the RUM collected data for blog.adobe.com, I identified two semi-related issues:

• On the home page (or really any place where the card block is used), viewmedia checkpoints are not sent because the resulting DOM uses style="background-image..." rather than an img element.
• On the home page and topic pages (or again, really any place where the article-feed block is used), viewmedia and viewblock checkpoints are not sent because the feed content is lazily loaded.

Note - I have opted here to bundle both solutions into a single JS file. These could certainly be broken apart but it seemed useful to bundle them into a single file containing any future RUM enhancements. But obviously open to feedback if maintainers would prefer an alternate approach.

[justinedelson]

Replaced with #37 because I had a slash in the branch name.