Admins create availabilities for their charity

https://trello.com/c/0qfz3zvZ/10-as-an-admin-i-can-create-availabilities-for-a-town-on-a-date

![image](https://cloud.githubusercontent.com/assets/8646429/5090022/179a56f4-6f0d-11e4-93ca-86144294908b.png)

The form will eventually be rendered in a `new.html.erb`, so the fail-redirect will be `render :new`. That's a few steps ahead and not a huge priority, so I am going to redirect in both success/fail cases.