Deep Link for Course Purchase Flow

[bonham000]

This PR:

• Add a /purchase deep link for linking into the app and starting the purchase course flow.
• Link accepts an optional courseId param which defaults to the TypeScript course.
• Example usage: https://app.pairwise.tech/purchase?courseId=fpvPtfu7s
• This link will start the payment flow which is described below.

Todo:

• [x] Purchase flow should continue after login?
• [x] SSO modal should dismiss correctly depending on initial app load state?
• [x] Validate the provided courseId param.
• [ ] Change/update messaging on the start payment modal? On the toast?

Notes:

The payment modal currently looks like this:

And the current payment flow is:

1. Create account
2. Start payment flow
3. Redirect through Stripe checkout
4. If no email exists on account, require email at step 2 before step 3
5. Payment succeeds and user returns to the app