Make import destination UI more intuitive and clearer

Make import destination UI more intuitive and clearer

Previously there were two fields: Destination namespace, and Destination
root page. They were both optional, and the "root page" one in particular
was a bit mysterious until you tried it out. In addition, there was a
strange interaction when you set both fields (I still don't quite
understand what used to happen in this case).

Now, there is a set of three clearly described radio buttons, allowing the
user to select whether to import pages into their automatically chosen
locations, into a single namespace, or as subpages of a given page. These
correspond to the three ImportTitleFactory classes available in MediaWiki.

See https://phabricator.wikimedia.org/M28 for a screenshot.

The logic of WikiImporter#setTargetNamespace is tweaked slightly to remove
the interaction between target namespace and target root page, since only
one of these options can now be set. Similarly, the API's import module
is modified in the same way.

Bug: T17908
Change-Id: I11521260a88a7f4a95fbdb71ac50bcf7b4fe5cd1