* Implement Azure AD oauth
* Use go-jose and cleanup
* Update go-jose in go.mod
* cleanup
* Add unit tests
* Fix scopes
* Add documentation page
* Improve documentation
* Convert extract_role into function.
* Do not use upn and replace unique_name with preferred_username
* Configure login button
* Use official microsoft icon and color from branding guideline.
* Add Azure AD config section in sample.ini.
GitLab could already be used as an authentication backend by properly
configuring `auth.generic_oauth`, but then there was no way to authorize
users based on their GitLab group membership.
This commit adds a `auth.gitlab` backend, similar to `auth.github`, with
an `allowed_groups` option that can be set to a list of groups whose
members should be allowed access to Grafana.
