Allow users to generate and save OAuth2 tokens

[nylen]

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 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