Add support for BillingPortal.Session

[moomerman]

This adds support for the Stripe Billing Portal Session.

https://stripe.com/docs/billing/subscriptions/integrating-self-serve-portal
https://stripe.com/docs/api/self_service_portal

[coveralls]

Coverage increased (+0.01%) to 89.623% when pulling b19846d on moomerman:feature/billing_portal into 0b941cd on code-corps:master.

[moomerman]

Depends on #602 being merged to fix the tests that break because of the updated stripe mock required for this new endpoint.

[johannesE]

I am in need of this feature. Thanks for the PR @moomerman .

If someone could merge this and #602 and release a new version on hex, that would be awesome.

[moomerman]

Since #602 was merged I've rebased and this is all good to go now 🚀

[snewcomer]

:yay

[lleger]

@snewcomer Any chance we can get a new hex release with this PR?

[johannesE]

While I would also love a hex release, I am using the workaround in my mix.exs: {:stripity_stripe, git: "https://github.com/code-corps/stripity_stripe", ref: "60d69dde7dc17670a043e5199d159dd0ee403437"},
@lleger you probably knew that you could do that. But if not, then this might also help you.

[lleger]

@johannesE Yep I'm doing the same in dev 😄 was hoping for a hex release before I ship to prod.