Default values for posts/topics fields on Category

When creating categories (or, at least, subcategories), certain integer
values are set to a default of NULL: topics_week, topics_month,
topics_year, posts_week, posts_month, and posts_year. This causes
consistent exceptions when trying to visit `/categories`, with the
offending line being in
`CategoryDetailedSerializer#count_with_subcategories`. This attempts to
coerce nil into Fixnum.

A fix could be to convert to 0 in the code, but these attributes should
really never be NULL. If there are no posts or topics, they should be 0
to maintain data integrity.

Signed-off-by: David Celis <me@davidcel.is>
Merge pull request #1913 from davidcelis/default-category-fields

Default values for posts/topics fields on Category