Categories: Don't suggest empty or old categories

The categories table holds all categories that exist OR have at one
point existed. This means it also contains a lot of vandalism and
other inappropriate names that happily come up in the suggestions.
This is undesirable. This changes the query to only list categories
with at least 1 entry.

Though technically it can be argued that an empty category with an
existing page might 'exist' and should thus be listed, with this
change they won't be. I don't think that is terribly incorrect to
say that such would be a 'new category', but we could also consider
renaming the label in the suggestion list to say:
'Currently unused category' or something similar.

As far as I can tell there is no way to test for page existance OR
generate a list based on allcategories with one API query.

Bug: 70025
Change-Id: I0e195d3ce26e69d4710d6a505b9da7d1b7678d92
Merge "Categories: Don't suggest empty or old categories"