Add MemberPress pause and resume capabilities

[remcotolsma]

MemberPress members may have the option to pause and resume their subscriptions themselves.

Some of the Pronamic Pay MemberPress gateways had also these capabilities:
836a3dc

Only after resuming, payments were also started for the period(s) between pausing and resuming. That's probably not how it works in the MemberPress PayPal and Stripe gateways.

Should we cancel the Pronamic subscription completely when pausing and to crate a new Pronamic subscription on resume? Or should we cancel/stop the current Pronamic subscription phase on pause and create a new Pronamic subscription phase on resume?

Stopping the current Pronamic subscription phase on pause and starting a new phase on resume is perhaps the clearest. We could even visualize the gap between resume and pause https://github.com/spatie/period style:

[rvdsteege]

I'd go with ending the current phase on pause and creating a new one on resume. This way it is clear (maybe in combination with a subscription note) why there have been no payments for a period, while the subscription seems active currently.