OAuth 2.0 authorization server

[vzickner]

This pull requests contains the implementation of the OAuth 2.0 provider.

Implemented authorization modes:

• client_credentials For admin authorization of other HelfenKannJeder services (especially user service)
• authorization_code This is the mode we use in come2help for Facebook and Google login. It is testable with the feature/oauth-helfenkannjeder branch of the come2help repository.
• password For direct login without see the authorization page, I think this is the way we should use for HelfenKannJeder services.
• refresh_token For refreshing the tokens

The endpoints are configured with the default Spring OAuth URIs. In the integration tests the usage of the endpoints is visible:

• /oauth/authorize
• /oauth/token

Additional to these endpoints there is a implementation of the oauth-provider-api, which allows the upcoming user service to manage users.

There are two clients configured until now, one for the user service (admin client) and one for come2help-web.