Bug 20744: Replace staff client header Koha logo gif with transparent png

Bug 20744: Replace staff client header Koha logo gif with transparent png

This patch replaces koha-logo-medium.gif with a fully-transparent png.
This patch also adds a white version in case someone finds it useful.

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

- View any page in the staff client and confirm that the Koha logo in
  the header area looks correct.
- Add the following to  IntranetUserCSS:

  .gradient {
     background: #46B9EA none;
  }

  - Reload the page and confirm that the Koha logo doesn't have a white
    fringe around the edges.
- Append this to the CSS you added to IntranetUserCSS above:

  h1#logo {
      background-image: url("/intranet-tmpl/prog/img/koha-logo-medium-white.png");
  }
  - Reload the page and confirm that the new white logo appears

Signed-off-by: Claire Gravely <claire.gravely@bsz-bw.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>