WIP: Add OAuth impl for social signin

[dstpierre]

This PR adds an OAuth implementation to enable sign-in with services like Google, GitHub, Twitter etc.

There's a new page in the web UI allowing to add a provider with the keys. The keys are encrypted.

It's TBD how the session token will be kept and how the JavaScript library will allow social login.

The current idea is to start the process like this:

1. The library will generate a unique key
2. This key will be pulled from the JS library until the sign-in process completes
3. Once completed, the library will receive a valid StaticBackend token, just like a normal email/password authentication.