Enable GitHub Organisation authentication for Grafana

We can enable GitHub Organisation authentication against a Grafana instance in order to allow access to the dashboards for the whole 2i2c GitHub organisation, or a community’s GitHub organisation.

To enable logging into Grafana using GitHub Organisations, follow these steps:

Create a GitHub OAuth application following Grafana’s documentation.
- Create a new app inside the 2i2c-org.
- When naming the application, please follow the convention <cluster_name>-grafana for consistency, e.g. 2i2c-grafana is the OAuth app for the Grafana running in the 2i2c cluster
- The Homepage URL should match that in the grafana.ingress.hosts field of the appropriate cluster support.values.yaml file in the infrastructure repo. For example, https://grafana.pilot.2i2c.cloud
- The authorisation callback URL is the homepage url appended with /login/github. For example, https://grafana.pilot.2i2c.cloud/login/github.
- Once you have created the OAuth app, create a new client ID, generate a client secret and then hold on to these values for a future step
Edit using sops the encrypted enc-support.secret.values.yaml file in the chosen cluster directory and add the credentials created in step one:
```
grafana:
  grafana.ini:
    auth.github:
      client_id: <client-id>
      client_secret: <client-secret>
```

Edit the support.values.yaml file in your chosen cluster directory and add the Grafana GitHub auth config, allowing the specific GitHub organization you wish to allow login.

grafana:
  grafana.ini:
    server:
      # root_url should point to the domain we redirect to if we have multiple
      # domain names configured and redirects from one to another
      #
      # FIXME: root_url is also required to be the same as the
      #        grafana.ingress.hosts[0] config specifically until
      #        https://github.com/2i2c-org/infrastructure/issues/2533 is
      #        resolved.
      #
      root_url: https://<grafana.ingress.hosts[0]>/
    auth.github:
      enabled: true
      # allowed_organizations should be a space separated list
      allowed_organizations: 2i2c-org