Fix line wrapping of text → HTML in ckeditor

While 2a2198bc fixed the display of the HTML → text conversion for
text/plain embedded in an HTML template, it failed to address the
ckeditor, which suffered from a similar problem.

Notably, the ckeditor formatted <pre><div>...</div></pre> as
<pre></pre><div>...</div>; while the <div> maintained its style, all
whitespace had been compressed from its original value, leaving the
style pointless.

This table shows the previous states of this code, and its failures:

   MARKUP           |     MUA      |  HTML->TEXT   |   CKEDITOR
  ------------------+--------------+---------------+---------------
   <pre>            | doesn't wrap | GOOD          | GOOD
     75cdfc91       | long lines   |               |
  ------------------+--------------+---------------+---------------
   <div style>      | GOOD         | whitespace    | whitespace
     6b117af0       |              | not preserved | not preserved
  ------------------+--------------+---------------+---------------
   <pre><div style> | GOOD         | GOOD          | whitespace
     2a2198bc       |              |               | not preserved
  ------------------+--------------+---------------+---------------
   <pre style>      | GOOD         | GOOD          | GOOD
     (this commit)  |              |               |

Swap to using <pre style="...">...</pre>, which MUAs and ckeditor render
acceptably, and the stock HTML → text conversion preserves whitespace
in.

Fixes: I#30198
Merge branch '4.2/ckeditor-text-wrapping' into 4.2.6-releng