Force about grouping for multi-element about groups

When converting an element that starts an about group with at
least one other element in it, we now only consider models that
support about grouping. This prevents the first node from being
converted to something else and leaving the others hanging.

In practical terms, this means that elements like
<link rel="mw:WikiLink/Category" typeof="mw:Object/Template">
get alienated and pull in the rest of their about group, rather than
being converted to a category or alienMeta or whatever and
leaving the other elements to be converted normally.

Added a test case that asserts this. Really the result should be an
MWtemplate rather than an alien, but that's a separate issue.
Also removed superfluous mustMatchAll checks; we've already
filtered the array by the time we get there.

Change-Id: I522ba4c56d5bc52c7e9aab1e2535385540c1315d
Merge "Force about grouping for multi-element about groups"