Add Oauth server support to allow ✨magical✨ ease of integration with other services

[autonome]

Purpose

I want to integrate 3rd party services and applications with Slate, in order to further reduce the barriers to usage.

Examples:

• IFTTT service: I started the integration, but account support would take a bunch of application glue code and the user experience would involve copy/pasting their Slate API key into a rando website. Oauth would make it familiar from a security UX experience, clean flow of solely clicking [yes], and vastly reduce the amount of code required.
• Browser extensions: I have a few extension ideas, one of which is a fork-and-go template for devs to make extensions which save various content verticals to Slate, or to add "backup to Slate" functionality to existing extensions. However, the same challenges arise as with the IFTTT example above.
• "Login with Slate" becomes an option - enabling 3rd party account identities to be anchored in Slate and service profile data to be stored there.

Deliverable

Slate is an Oauth server.

Dependencies

N/A

[jimmylee]

We are planning to do this right after we support adding e-mails (securely)

Our current list involves Twitter and maybe Google. Happy to do this and treat Slate as an oauth-identity-service :)

[autonome]

Could short-circuit a way of doing this for things like the IFTTT integration by putting key in the URL instead of headers.