Bug 26949: Upgrade TinyMCE in the staff interface from 5.0.16 to 5.9.2

Bug 26949: Upgrade TinyMCE in the staff interface from 5.0.16 to 5.9.2

This patch upgrades TinyMCE in the staff interface from 5.0.16 to 5.9.2.
Other than the required TinyMCE package files, the only other change is
to the internationalization include file, which is updated to include
the latest set of strings for translation.

To test, apply the patch and clear your browser cache if necessary.

Test that the TinyMCE editor works in the staff interface:

 - In system preferences, with the  UseWYSIWYGinSystemPreferences
   preference enabled: IntranetCirculationHomeHTML,
   IntranetmainUserblock, IntranetReportsHomeHTML, etc.
 - Additional contents, with the AdditionalContentsEditor preference
   set to "WYSIWYG."
 - Administration -> Libraries, in the "OPAC info" field.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>