Update membership status on subscription status change

[rvdsteege]

If the Pronamic Pay subscription status changes, this doesn't update the membership status in Restrict Content Pro. A chargeback results in the subscription status being put to 'On Hold', but this is for example not reflected in the RCP membership.

Internal Help Scout ticket: https://secure.helpscout.net/conversation/2519635424/26936

[remcotolsma]

Added some feedback in 7fe6938.

[rvdsteege]

Made some changes in https://github.com/pronamic/wp-pronamic-pay-restrict-content-pro/compare/7fe6938…43a2db0 and replied in 7fe6938.