Allow org-specific endpoint to be specified for Azure OAuth provider

[raganca]

Feature request

• See title.

Is your feature request related to a problem? Please describe.

• The OAuth auth and token URLs are hardcoded to the common endpoint, which requires that an app registration on Azure be set to multi-tenant.

Describe the solution you'd like

• A field in the External OAuth Providers -> Azure Enabled options which allows the Azure tenant ID to be specified. This would replace the 'common' in /common/oauth2/v2.0/authorize with a UUID representing a specific tenant.
• Alternatively, could allow user to paste in the whole endpoint URL, but the additional effort may not be worthwhile.

Describe alternatives you've considered

• There's not a clear alternative to this in my mind, but I'm certainly open to suggestions!

[koba-ninkigumi]

There is already a pull request for this issue. #360

Once this is merged, this feature will be accomplished.

cf. https://docs.microsoft.com/en-us/azure/active-directory/fundamentals/auth-oidc