OAuth Sign in and Sign up flows

- [x] Clean up sign in view.
- [x] Fix linting errors.
- [x] Handle error conditions.
- [x] Add unit tests.
- [ ] Add functional tests.
- [x] Add unit tests for assertion.js.
- [x] ~~Externalize jwcrypto~~ (#960).
- [x] Fix jwcrypto related errors in mocha tests (#961).
- [x] Add sign up