Allow Omniauth providers to be marked as external

Related to #4009 (closed)

With this MR we will be able to allow the user to set which Omniauth Providers they would like to have as external. All users login in via these providers will be marked as external, even if they already had an account before. If the provider is removed form the list of external providers, the users will be marked as internal at their next login.

MR for Omnibus: omnibus-gitlab!727 (merged)

/cc @dblessing @DouweM