Add further tests for creation of the internal admin body

The internal admin body should be created automatically if it
doesn't exist on calls to PublicBody.internal_admin_body; we've
seen errors (#1001) where this fails after the default locale
changes.  (Although these tests don't actually replicate that
problem.)
Make test fail as with issue #1001

This requires us to clear the I18n.fallbacks when changing the
I18n.default_locale, otherwise the old default locale remains there.
Ignore translations when finding the internal admin body

If you change the default locale after having set up your site,
and exception would always be thrown by
PublicBody.internal_admin_body, which looks up that distinguished
body based on url_name.  Since that's a translated field, and
there would be no translation for it in the new locale, it
wouldn't be found, and the code would try to create a second
internal_admin_body, which would fail because of the url_name
being non-unque in public_bodies.

There are various ways of fixing this; the one introduced in this
commit is to use raw SQL to find the public body, bypassing the
public_body_translations table.

Fixes #1001.