Bug 26942: Allow style and link tags in NewsEditor TinyMCE

To Test:
-Make sure the sys pref 'NewsToolEditor' is set to TinyMCE
-Go to any news item and add this line:  <link href="https://fonts.googleapis.com/css?family=Open+Sans" rel="stylesheet">
-Save the news item
-Re-open the same news item again in TinyMCE.
-The editor has removed that line. Annoying!
-Apply patch
-Try again, the <link> should stay now

Signed-off-by: Amit Gupta <amitddng135@gmail.com>

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

koha-tmpl/intranet-tmpl/prog/en/modules/tools/koha-news.tt

@@ -400,6 +400,8 @@ Edit news item[% ELSE %]Add news item[% END %][% ELSE %]News[% END %]</div>
                 menubar : "file edit view insert format tools table",
                 mode : "specific_textareas",
                 plugins : "autoresize table hr link image charmap lists code emoticons",
+                extended_valid_elements:"style,link[href|rel]",
+                custom_elements:"style,link,~link",
                 toolbar : [
                     "formatselect | bold italic | cut copy paste | alignleft aligncenter alignright | outdent indent | image link unlink anchor cleanup hr",
                     "table | bullist numlist | undo redo | removeformat | emoticons charmap | forecolor backcolor | code visualaid help"