Fix bug 850656: Set based_on field correctly

This tries to make sure that the "New:" keyboard in the revision dashboard is (more) correct.

1) Don't mark **page moves** as new (dashboard/views.py):
I'm just checking if rev.document.is_redirect.

2) Don't mark **reverts** as new (wiki/models.py):
If in the default locale, set based_on to the revision we reverted from (it's null currently, that's why they appear as *New:*). Leave translations alone, as they always refer to a revision of the default language document.

3) Don't mark **all translations** as new (wiki/views.py):
This looks really unfortunate :( We forgot the document instance for the validation and so there is no based_on reference for translation revisions at all. That information is badly needed to compare against the English version (once we have a l10n dashboard, we will miss this data :-/)

Not yet resolved:
As said in 3) all translation revisions will need the based_on reference and so does the very first revision of a translation. That means that new translations aren't marked as *New:* currently. (Maybe count that the revision is the only one for a document?)