LMS-1963

Now any of the plurals being missing will count as an Empty translation,
and for looking at tags, the two English strings and all the translated
strings are mooshed together.

The tests in test_validate.py now examine the messages to see that they
have the right structure.  The English and the translation should have
the same tags, the translation can't be blank, and it can't have any
Astral plane characters.