Implement "action" argument for authorize endpoint

[toolness]

On the teach site, there are two different ways a user could end up at the oauth2 authorize endpoint on id.webmaker.org:

• By clicking the "login" link, in which case the user expects to be prompted for their username/password.
• By clicking the "sign up" link, in which case the user expects to sign up.

I'm thinking that this intent could be conveyed by an intent argument to the authorize endpoint. For example, intent=login could indicate to the authorize endpoint that the user wants to login, while intent=signup could indicate that the user wants to sign up.

I'm thinking it could be the burden of the oauth2 client (not id.webmaker.org) to keep track of what the original intent was when the oauth2 callback is triggered. In other words, id.webmaker.org doesn't need to pass intent back to the callback when the user is done logging in or signing up.

This should be documented in docs/oauth.md too.

[toolness]

Alternatively, we could call the argument action and do things the exact same way as the fx accounts folks: https://github.com/mozilla/fxa-oauth-server/blob/master/docs/api.md#request-parameters-4

[toolness]

Ok, we added documentation for this in #189, and I added teach-api support in mozilla/teach-api@d8812c7 and teach.webmaker.org support in toolness/teach.webmaker.org@247170f. The front-end with OAuth2 enabled is (temporarily) at http://mozteach.toolness.org/, so as soon as things are deployed to id.mofostaging.net, everything should work!