Fixes T1731: code in iframes renders with white-space: normal

Summary:
Fixes T1731
Chrome by default on a user-agent stylesheet sets white-space to `pre`.
For `pre` elements. This is useful in most cases since whitespace is taken
literally.

Unfortunately when people send out emails with code in them, the `pre`
blocks can get mangled because there may be unnecessary whitespace in
them.

I personally think the default behavior is much better, however, I think
we should have `pre` blocks render with `white-space: normal` because
anyone who's sending an email with code it in will probably use <span> and
<br> and inline styles to ensure it looks correct on most readers. Since
many places (like Outlook) strip whitespace out of HTML, I think we should
mimic that behavior too.

Test Plan: edgehill --test

Reviewers: bengotow

Reviewed By: bengotow

Maniphest Tasks: T1731

Differential Revision: https://phab.nylas.com/D1561

internal_packages/message-list/lib/email-frame.cjsx

@@ -62,6 +62,10 @@ EmailFixingStyles = """
     -webkit-font-smoothing: antialiased;
   }
 
+  pre {
+    white-space: normal;
+  }
+
   a {
     color: #2794c3;
   }