MBL-1159: Exchange temporary token for OAuth token and login

[amy-at-kickstarter]

📲 What

This implements the MVP of login with OAuth.

It works like this:

1. Open a web authentication session
2. If it completes successful, the website redirects to a URL which includes a tempoary authorization code
3. We exchange that code and a secret for the real OAuth token
4. We use that OAuth token to fetch the user's information
5. ...and then save that to the app environment login.

🤔 Why

We are porting the app to use OAuth via a web authentication session to improve security.

👀 See

This snazzy demo!

https://github.com/kickstarter/ios-oss/assets/146007185/19803d96-04c0-4e1c-b596-c9527bc63d8e

[amy-at-kickstarter]

Converting to draft while I write some tests.