3535 Separate subscriptions by type

https://code.google.com/p/otwarchive/issues/detail?id=3535

Updating https://github.com/otwcode/otwarchive/pull/1144
So this is just a little more verbose than it needs to be, and it would be better to do the filtering as part of the database query instead of getting them all back and then whittling down the array, which is what find_all is doing here. You could do something more like:

```ruby
def index
  @subscriptions = @user.subscriptions.includes(:subscribable)
  if params[:type].present?
    @subscriptions = @subscriptions.where(subscribable_type: params[:type].classify)
  end
  @subscriptions.to_a.sort! { |a,b| a.name.downcase <=> b.name.downcase }
  @subscriptions = @subscriptions.paginate :page => params[:page], :per_page => ArchiveConfig.ITEMS_PER_PAGE
end
```

And it doesn't need to be part of this pull request, but we might want to consider storing the sort criteria in the subscriptions table, so that we're not getting all the subscriptions, sorting them, and then paginating.