WebAccess: OpenID and OAuth authentication
- Introduces OpenID and OAuth external authentication plugins.
- Support out of the box several authenticatio providers (including Google and Facebook). (addresses #1124)
- Merge note: this commit is exactly the one provided by Alper. All amendments will be available in the next commit.
Reviewed-by: Samuele Kaplun <samuele.kaplun@cern.ch>