Add dependency on the org.sweble.wikitext.swc-engine artifact.
Add dependency on the org.wikimedia.mwdumper artifact.

This is not available in the central Maven repository, so before
attempting to build with this dependency it's necessary to first
git clone https://git.wikimedia.org/git/mediawiki/tools/mwdumper.git
and then mvn install this manually.
Create mediawiki.SiteImporterImpl to import MediaWiki.

This code badly needs a refactoring. SiteImporterImpl is now a behemoth
containing inner classes doing most of the work, using the mwdumper and
wikitext libraries, while calling reuseable functionality from
com.google.sites.liberation.imprt to push stuff to Sites ...

The implementation of the UI now also sucks and someone (preferrably not
me) ought to fix it. Having the mediawiki package depend on the
imprt.SiteImporter interface and making mediawiki.SiteImporterImpl
public is just icky, but it's a very quick way to make this work at all.

Well, it's a start.
Merge remote-tracking branch 'remotes/local/implement-mediawiki-support' into implement-mediawiki-support