Fix munging of linked external images

htmLawed stripped/discarded <div> tags nested inside inline tags such as <a>. The previous replacement engine would replace <img> tags with <div> tags and would cause the HTML to be corrupted by htmLawed.

This patch uses <span> tags which have CSS rules to be displayed as inline-blocks. This should stay truer to the normal sizing and placement of the original <img> tag.

References:
http://osticket.com/forum/discussion/77309/osticket-1-9-poor-html-handling