Bug 26419: Replace OPAC Koha logo with SVG

Bug 26419: Replace OPAC Koha logo with SVG

This patch replaces the OPAC Koha logo PNG file with an SVG version.

To test, apply the patch and rebuild the OPAC CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

 - Clear your browser cache if necessary.
 - View any page in the OPAC. The Koha logo in the upper-left corner of
   the screen should look good.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>