Fallback to screen.availHeight for iframe bug (#579)

What changed?
----------------

This commit uses the `screen.availHeight` property as a fallback 
to how we currently set the iframe's height. See [screen docs]. 

[Screen docs]: https://developer.mozilla.org/en-US/docs/Web/API/Screen

Why?
-------

As part of the Wallaby feature test I've needed to read an email, 
but I could only get iframe element using 
`Wallaby.Query.css("iframe", visible: false)` because the height 
of the element is 0.

I've tried this in multiple versions of Chrome and chromedriver. 
Also, I've only been able to experience this in headless Chrome 
in macOS. I wasn't having this issue with Wallaby option 
headless: false nor on a CI instance running Ubuntu.

Related to #350 and 
#351

lib/bamboo/plug/sent_email_viewer/index.html.eex

@@ -233,7 +233,10 @@
             <script>
             function adjustFrameHeight(iframe) {
               var contentWindow = iframe.contentWindow;
-              iframe.style.height = (contentWindow.outerHeight) + "px";
+              var height = (contentWindow.outerHeight)
+                ? contentWindow.outerHeight
+                : contentWindow.screen.availHeight;
+              iframe.style.height = height + "px";
             }
             </script>
             <iframe onload="adjustFrameHeight(this)" src="<%= "#{@base_path}/#{Bamboo.SentEmail.get_id(@selected_email)}/html" %>"></iframe>