We pull the list of App IDs in `upsertApp` to see if we already created
the App ID. This broke when the user had too many App IDs, since DS
returned a paginated response and we only looked at the first page.
We could paginate the whole list (as seen in #89) but that's slow.
Instead, just filter the query to the expected App ID. This is a prefix
search so we might still receive multiple responses, but it's much less
likely that the user has like 10(? I'm not sure what the default/max
pagination limit is) apps registered with the XTL-ABC.com.foo.bar prefix
(they could have PlugIns but usually just a small number of them.)
This is a tentative fix for #87.