Allow setting a date when changing from a paid membership to a free membership

Membership are required for players to do anything in CR. Because of that, we need to be able to select a date for our players when they are downgrading from a paid membership to a 'free' membership. Right now we cannot select a date - if you change the membership to the free option, it kicks in the day you set it. 

We need to be able to 'cancel' the paid membership at the end of the month (we can do that now) AND set the 'free' membership to start as well. Right now, the players whose memberships are cancelled then are left with no membership, and we have to run a report and manually go in and set them to our 'free' membership which is time consuming and an inconvenience.