Configure OAuth2 Providers

[simonoakesepimorphics]

• Added the ability to configure multiple OAuth 2.0 and OpenID Connect providers.
• Added the ability to enable / disable OAuth providers.
• Enhanced error messaging when user registration or login fails.

Note that the changes in this branch correspond to the oauth2 branches in registry-config-base (pull request) and registry-core.wiki.

[marqh]

I agree that this change looks suitable, in conjunction with https://github.com/UKGovLD/registry-config-base/pull/10

I am happy for this to be adopted