widgets: flatten transparent background to white

OH_Partner_frame.png has an alpha channel — the right-side content area
where name, commits, and kudos are composited was transparent rather
than opaque white. The kudos laurel badge composited on top has a white
background, making the two visually inconsistent: the laurel shows white
while surrounding areas are transparent, appearing as a checkerboard in
image viewers or taking on the page background color in browsers.

Add image.flatten to setup_blank in badge_helper.rb. flatten fills
transparent pixels with the current background color, which is already
set to white on the line above, producing a fully opaque image before
any compositing begins. setup_blank is shared by all three badge types
(Account, Partner, Thin), so all affected test fixtures are regenerated.

Author: mattst88

lib/widget_badge/badge_helper.rb

@@ -15,6 +15,7 @@ module ClassMethods
       def setup_blank
         image = MiniMagick::Image.open(IMAGE_DIR.join('OH_Partner_frame.png'))
         image.background('white')
+        image.flatten
         image
       end