Fix image CID rewrite for some pasted images

Copy+Pasting images from a ticket thread or from a FAQ article to the HTML
editor and submitting will result in the URL *not* being properly rewritten
for storage in the database. When images are uploaded, a relative URL is
returned from the server and embedded in the HTML editor. However, when
images from the system are copy and pasted into the editor, an absolute URL
with the http:// scheme is included. This URL was not properly handled by
the URL rewriter in Format::localizeInlineImages().

This patch addresses the issue by properly handling both relative URLs from
image uploads and absolute URLs from copy and pasting.