org.el: Don't activate a plain link when it is part of a bracketed link

org.el: Don't activate a plain link when it is part of a bracketed link

* org.el (org-activate-plain-links): Don't activate a plain
link when it is part of a bracketed link, unless bracketed
links are not enlisted in `org-activate-links'.
(org-open-at-point): Don't consider the text immediately after
a bracketed link is part of a plain link.

This fixes a bug when fontifying [[http://orgmode.org][Org]]Mode --
where "Mode" should not be fontified, because it is not part of the
bracketed link.

Note that there was another related bug: C-c C-o on "Mode" used to
try opening a plain link.  Also fixed in this commit.