Update CHT login page to have button for redirecting to OIDC provider

[jkuester]

• When an OIDC provider is configured for a CHT instance, the logic page should show a button to allow the user to "Login with SSO". Selecting this button should cause the user to be re-directed to the SSO login page.
• Get the redirect URL from openid-client (might be the buildAuthorizationUrl function?)
  • Consider making an oidc service in the API to manage the openid-client functionality
  • The client_secret for communicating with the OIDC provider should be stored/loaded as a CHT credentials value. See the outbound push password_key functionality for an example of how to do this.
• When a user visits /medic/login, the renderLogin function in api/src/controllers/login.js should serve the login page with the redirect button enabled/disabled based on config.
• Can add integration test(s) for the login controller
  • For the integration tests, I think we will have to consider if we need to spin up an OIDC provider, or if we can just create a fake one...