Remove <base> tag from HTML in articles.

Fixes bug#8880 - No inline image shown if HTML email contains 'base' tag
http://bugs.otrs.org/show_bug.cgi?id=8880

The <base> attribute breaks any <img> tags pointing to relative sources.
This is the case in some mails generated by MS Outlook. In that case
the inline images do not show.
Merge remote-tracking branch 'upstream/rel-3_2' into rel-3_2