Bug 26942: Allow style and link tags in NewsEditor TinyMCE

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>
(cherry picked from commit 27fce8b0c2575021400023d9a7460adef20dadd6)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>