Pause Membership Improvements

To simply the pause membership feature we plan to make the following improvements:

Rather than calculating complicated pro-rated amounts every time a membership is paused/resumed we will change this so when a membership is paused we will just push the next billing date out by the equal period of time, this will bring several benefits that we can then quickly implement:

1. Pauses can be flexible for any period, rather than fixed periods

2. We can enabled extensions to existing paused memberships easier

3. It would support and make it easier to build Pause Limits feature

4. It would enable ability to pause pre-paid memberships (by pushing expiration dates instead)