Add MemberPress to options scrublist

[klasharr]

MemberPress has its own payment gateways which will need severing on non-prod environments, and emails need to be disabled.

Here are some docs from MemberPress on setting up staging envs: https://docs.memberpress.com/article/367-how-to-create-a-staging-site-with-memberpress

[klasharr]

FYI @georgestephanis ^^