Don't replace !! in elements

Don't replace !! in elements

 * 55313f4e almost got it right, but missed the str_replacing table
   headings.

 * Thankfully, this was doubly broken before that patch since the
   StringUtils::explodeMarkup would have skipped the || which would
   go on to be explode by table cell attribute parsing. The test case
   provided would look like,

   <table>
   <tr>
   <th class="">|">ha</div> ho
   </th></tr></table>

   Suffice it to say, noone is using this in production.

 * Note that we can't just entity encode the ! since that would break
   style attributes with !important.

 * Also note, Parsoid already gets this right.

 * Adds a StringUtils::replaceMarkup

Change-Id: Iab3ae4518fcb307b795d57eece420ba48af0a3bf