Fix a few sorting issues

Currently forum catgories are sorted "position DESC" in ForumsController#index, which is the opposite from the default forum sorting. Also, forum position can't be set on the forum form in the admin interface, since :position is not permitted.

This commit does this:
* default Category sorting to "position ASC", same as with the Forum model
* allow forums "position" to be mass-assigned
I'm always wary of `default_scope` usage... could we make this a scope instead and say `Forem::Category.by_position` instead? Sorry to be a pain.