Currently, the default sort+period combination is "by activity, last 3
days". This works fairly well on the home page, but isn't working well
inside individual groups because there usually aren't many posts made
yet in a specific group in the last 3 days. Because of this, going into
an individual group looks quite empty by default.
This commit changes the default *only* when inside a group to be "by
activity, all time". This will still be overridden if the user has set
up custom defaults.