Finish off ORCID integration via django-allauth

The new official version of django-allauth includes the work I have done to integrate ORCID as social
provider. With this commit django-allauth is setup in the codebase.
Fix a bug related to the regular signup (not using social providers)
Update the doc about Social authentication: add ORCID
Merge pull request #257 from nimiq/orcid

Orcid