Allow comped members to upgrade as paid member to tier

[rshbhgrg]

refs https://github.com/TryGhost/Team/issues/1068

At the moment, complimentary members are not able to change their plan. They cannot pay nor can they cancel. This change allows comped members to be able to upgrade as paid member via stripe checkout on same or different tier -

• allows comped members to create stripe checkout session
• updated attached tiers to a complimentary member on upgrade

Note: This change does not cover cancellation of comped stripe subscription at the moment, i am looking into it.