Handle aliens correctly in the converter
Previously, data-mw-gc (generated content) elements were unconditionally
converted to alienInline nodes, and unrecognized elements were
unconditionally converted to alienBlock nodes. This is wrong and
produced weird results when I started experimenting with <code> tags.
Instead, I made both gc and unknown element trigger alienation, but the
decision of whether we generate an alienInline or an alienBlock node is
separate and is based only on whether we're inside a content node.