Move default logo to resources/assets/ directory

The problem here is that the path to 'wiki.png' is saved in users'
LocalSettings.

We likely should not remap the path, like we did for footer license
icons in Ic7c32e56:

* It's likely that users changed their logo image by overwriting the
  file in skins/common/.
* If the normal upgrade process is followed (overwrite-uploading new
  files), the old file will still be there with the skins/common/
  directory.
* If it does cause problems, they'll be rather easy to notice and fix.

On the other hand, maybe we should?

* This is going to be annoying for git users.
* It will bite anyone who deletes all MediaWiki files when upgrading
  via tarball, which is more likely with the recent skin system
  changes encouraging users to remove old cruft from skins/ directory.

Bug: 69277
Change-Id: I175fe57048ebf9d348fb2fe67bf62cf5df389003