Allow users to generate and save OAuth2 tokens

Let's add a feature to this version of the console that allows users (on WP.com) to choose one of their applications configured at https://developer.wordpress.com/apps/, generate OAuth2 tokens, and save them.

Currently we don't provide an easy way for users to do this - we expect them to implement [our OAuth2 flow](https://developer.wordpress.com/docs/oauth2/) themselves. GitHub and Twitter make this much easier, and it would be a really good usability improvement for our API.

Previous discussion:

- p3hLNG-IW-p2
- p3hLNG-gs-p2

Start by reading the WordPress.com OAuth2 documentation and the application-management page linked in the issue, then inspect the React/Redux console entry points. The feature is done when a WP.com user can choose a configured application, generate OAuth2 tokens, and save them for use in the console; the previous discussion links may provide context.

Written by the indexing model from the issue text.