feat(k2): Wrap lines on preformatted text

Summary:
We now use <pre> tags to preserve whitespace in plaintext emails,
but it's annoying when there are long lines that force a scroll
bar. Wrap them instead.

Test Plan:
looked at some emails. Not sure this isn't going to break something
           weird---is <pre> used in anything else?

Reviewers: bengotow

Differential Revision: https://phab.nylas.com/D3484

src/K2

@@ -1 +1 @@
-Subproject commit f0b15bcf27f215cb1781fec75284b8d82ba64f3a
+Subproject commit 36fe5050cd5d8c644adf5246d601cd829909b3a5

static/email-frame.less

@@ -99,6 +99,11 @@
     max-width: 100%;
   }
 
+  pre.nylas-plaintext {
+    white-space: pre-wrap;
+    word-wrap: break-word;
+  }
+
   img {
     border: 0;
   }