Replies with the rich text should quote an HTML part using <blockquote>

The gmail_quote class is semi-standard, and occasionally aids in
display.
Ensure that plaintext signatures are escaped correctly before being appended

We use specific escaping, rather than EscapeHTML, because the
signature-detection code in RT::Interface::Web::StripContent assumes
that the signature was transformed into HTML by CKEditor; as such, to
ensure that no-data messages are detected as before, use the same
transformations here.
Logic simplification
Add styling hints to <blockquote>-quoted sections
More liberal signature detection in HTML mail

Some examples of signature insertion in various clients:

GMail:
  [...]</div>-- <br><div dir="ltr">This is my <b>bold</b> signature!</div>

Thunderbird:
  [...]
  <div class="moz-signature">-- <br>
    This is my <b>bold</b> signature!</div>

Apple mail does not insert sigdashes by default.  When added by hand,
they are implemented via:
  [...]</div><br><div>
  <div style="[snip]"><div>--&nbsp;</div><div>This is my<span class="Apple-converted-space">&nbsp;</span><b>bold</b>&nbsp;signature!</div></div>
  </div>
...which is difficult to catch without false-positives.

Unfortunately, Evolution uses:
  [...]<BR>
  <TABLE CELLSPACING="0" CELLPADDING="0" WIDTH="100%">
  <TR>
  <TD>
  -- <BR>
  This is my <B>bold</B> signature.
  </TD>
  </TR>
  </TABLE>
...which is much more difficult to extract, as the whole table would
need to be stripped.
Trim HTML in replies to the contents of the <body> tag

Email clients vary as to if they attempt to provide a full HTML document
(starting with doctype and <html>), or merely an HTML-formatted snippet.
Content which contains an <html> tag unfortunately interacts poorly with
CKEditor when shoved inside a <blockquote>, effectively imediately
closing the blockquote.

Pass content to quote through an HTML scrubber to remove the offending
<html> tags, the <head> tag, and and any <meta> contents thereof.
Ensure that "Show quoted text" always displays on its own line
Merge branch '4.2/reply-to-html' into 4.2-trunk