add paying subscription scope to active admin

we've added two scope in here
1) Subscription - this is all groups who have elected to pay a subscription for Loomio
2) Paying subscripions - this is all groups who have also selected a subscription plan

note there is currently nowhere in the admin panel to see the amount/ size of a subscription
![image](https://f.cloud.github.com/assets/2665886/914670/2df4aa52-fe44-11e2-9669-da4e253c5e53.png)