Method to find the siblings of an RT::Attachment

Siblings are attachments on the same MIME part level as each other.

This is especially useful for children of multipart/related parents,
since the first child usually embeds its siblings in its content.
Method to find the nearest ancestor of an RT::Attachment with a certain MIME type

The nearest matching ancestor, or closest containing part, is
particularly useful when finding the enclosing multipart parent for
further processing.
Display embedded images in HTML messages inline with the text

Attached images with a Content-ID header referred to by an <img
src="cid:..."> tag in the HTML are now properly displayed as long as the
existing $ShowTransactionImages option is enabled.  If the option is
disabled, images are not inlined with the HTML.

This makes RT's message display even more like a real mail client's.
Move the HTML::RewriteAttributes dependency to CORE from DASHBOARDS

It's unlikely anyone configures RT with --without-dashboards, but just
in case.
Explain why attached images aren't shown if ShowTransactionImages is off

This is similar to our behaviour with SuppressInlineTextFiles and more
obvious what's happening.
It's not a message body, it's an image
Tidy up indenting and whitespace

Whitespace only changes, no functional difference.
Rewrite inline images when serving a single HTML attachment for display

This only happens in the non-default configuration of
AlwaysDownloadAttachments off (default) and TrustHTMLAttachments on
(non-default).  The latter option is primarily used by people so HTML is
rendered in full without the restrictions imposed within ticket history.
It's reasonable to surmise that a site already trusting HTML attachments
for better display will prefer working embedded images.
Preserve the Content-ID of redistributed attachments

This opens the door for supporting inlined images in HTML notifications.
Don't linkify URLs in HTML src attributes

Yet another problem stemming from MakeClicky treating HTML like plain
text.  Unfortunately fixing that requires a much larger rework, for a
separate branch.
Optionally show remote images in HTML messages

Display is off by default for security and privacy concerns, but it is a
useful option for folks who frequently receive HTML mail.
Merge branch '4.2/inline-images'