Make external image placeholder consistent

And use admin-defined site logo
Fix external image in an inline <a> element

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.
Fix placement of [Show Images] button for clients

Previously, the display of the button was incorrect and more than one button
might have been displayed.
Merge pull request #936 from greezybacon/issue/linked-ext-images

Fix munging of linked external images

Reviewed-By: Peter Rotich <peter@osticket.com>