web-user-interface: Addition of new icon for secure connection certificate download

web-user-interface: Addition of new icon for secure connection certificate download

- This uses a padlock icon from https://commons.wikimedia.org/wiki/File:Encrypted.png
- The license for this image is the following:-
   This library is free software; you can redistribute it and/or modify it under the terms
   of the GNU Lesser General Public License as published by the Free Software Foundation;
   either version 2.1 of the License, or (at your option) any later version. This library
   is distributed in the hope that it will be useful, but without any warranty; without
   even the implied warranty of merchantability or fitness for a particular purpose. See
   version 2.1 and version 3 of the GNU Lesser General Public License for more details.
- Based on the above license I believe it can be used by IPFire covered by the GNU General
   Public License that is used for it.
- The icon image was made by taking the existing openvpn.png file and superimposing the
   padlock icon on top of it at a 12x12 pixel format and naming it openvpn_encrypted.png

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>