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>
Don't notify about dominating private messages.
Merge pull request #1917 from awesomerobot/master

fixing watermark/gutter link interference
Merge pull request #1916 from vikhyat/pm-dominating

Don't notify about dominating private messages
Be slightly less aggressive at clearing assets
BUGFIX: make sure moderators can't view topics they aren't allowed to
Client and Server side routes for a Group
More templates for the group page
Move `Discourse.Group` into public models
Return a promise from `Discourse.Group.findAll()`
Rename `load` to `loadUsers` which is what the method was actually
doing.
Work in progress: Groups Page
Can list group members
Link to groups from user profile
FEATURE: First stab at groups page with member list
Added some specs for the group controller methods
Merge pull request #1913 from davidcelis/default-category-fields

Default values for posts/topics fields on Category
FIX: more protection from nil in count_with_subcategories
Small tweaks to Group page
FIX: Component was not refreshing all the time