Don't display broken, unparsed comments in client recent changes

If $comment is a string, then it is probably the new comments
format that we will eventually parse and display nicely. This
is not implemented yet and we shouldn't pass these yet to
the ExternalComment objects.

afaik, other comment strings are not used anymore and just
defaulting to the generic 'wikibase-comment-update' is okayish
in this case and is what we want for now.

Added details on how this all works in code comments.

Also added a lot more test cases to ExternalChangeFactoryTtest

This catches this bug and covers other scenarios.

Bug: T110823
Change-Id: I343acd5c755f9fb9480d2280982b54df9695a137
(cherry picked from commit 25a164ca610c2e04046773c5d3c534a2b3cf1f02)