(bug 48236) Update login.wikimedia.org logo

This commit replaces the current login.wikimedia.org logo
with a GIMP-created and losslessly compressed version that
does not have rsvg's rendering issue mentioned in the bug.

The file is fully protected on Commons and weighs just
3273 bytes, which is quite good for a logo.

Bug: 48236
Change-Id: I43ffba30080589705e231c385911460049e0ecb5