Add test endpoint and ability to add tokens from backend

[dshanske]

This adds two features

• Ability to add a custom token from the admin. This allows for it to be given to systems that can't support a token exchange workflow and for testing purposes.

• A test endpoint built in for troubleshooting purposes. This is part of the attempt to diagnose problems with sites that aren't authenticating. The test endpoint basically tests the 'determine_current_user' flow by either returning an error or returning a successful token.

[dshanske]

@pfefferle I made the requested changes.

[pfefferle]

cool, thanks.