Suppress some images in z_url_metadata

[mworrell]

While fetching metadata for https://www.erlang-solutions.com/blog/the-top-10-elixir-talks-of-2017-so-far.html

We find an image with url

https://www.facebook.com/tr?id=18351324...&ev=PageView&noscript=1

This is obviously a tracker image and should be suppressed by our routines.

The surrounding html was:

<noscript><img height="1" width="1" style="display:none"
src="https://www.facebook.com/tr?id=18351324...&ev=PageView&noscript=1"
/></noscript>

The real blog post image is a bit hidden as a background image:

<div class="big-image"
          style="background-image: url(https://esl-website-production.s3.amazonaws.com/uploads/blog_post/image/105/Elixir_top_10_Blog.jpg);">
</div>