Bug 11584: Add wysiwyg editor to system preferences dealing with HTML

Bug 11584: Add wysiwyg editor to system preferences dealing with HTML

This patch adds the ability to use a WYSIWYG editor for system preferences.

The key files that I touch are:

1) admin/systempreferences.pl
2) koha-tmpl/intranet-tmpl/prog/en/modules/admin/preferences.tt
3) koha-tmpl/intranet-tmpl/prog/en/modules/admin/systempreferences.tt

I also add:

4) koha-tmpl/intranet-tmpl/prog/en/includes/wysiwyg-systempreferences.inc

and

5) koha-tmpl/intranet-tmpl/lib/tiny_mce/plugins/advimage

This plugin is part of the TinyMCE distribution. It used to be in Koha, but
then someone removed it. It's useful for preferences like "opacheader" though.

*If you're using anything except IE, this should work super well. If
you're using IE, it'll probably only work for keyboard input and dragging
text within the editor box but not from outside of it. IE has worse
security, so you can probably paste using the context menu paste.

*While I think a WYSIWYG editor can be useful, there might be times
where the content is displayed differently than it is in the editor
because of higher level CSS and Javascript.

Signed-off-by: Martin Persson <xarragon@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>