Use a custom MediaWiki logo during installation

Use a custom MediaWiki logo during installation

Previously we used the regular logo, which could prove confusing when
one switched back-and-forth between the installed and mediawiki.org
pages.

The new one is "half-drawn", evoking the image that we are in the
process of creating something (a new wiki). I like that.

Source: <https://commons.wikimedia.org/wiki/File:Wikitech-logo.png>,
authored by Brion, GFDL 1.2+. It was previously used as a logo of the
Wikitech wiki, but that has unicorns instead now.

Change-Id: Ic5c83bb85e6b6a11978d452fbe46d2dad9c5a553