Clean up OIDC authorization screen

[allanlasser]

1. Updates copy
2. Adds margin around card and adjusts size
3. Stretches buttons to full width

Test steps

1. Create a preview deployment in Heroku
2. Go into the Django Admin and create a new OIDC Client. Select a code auth type. Give the new client an arbitrary redirect URI, like http://localhost:8000/mock/callback. Otherwise use default values.
3. Copy the Client ID from the created OIDC.
4. Navigate to {previewDeployUrl}/openid/authorize?response_type=code&client_id={clientId}&redirect_uri={redirectUri}&scope=openid, providing the client ID and redirect URI from steps 2 & 3.
5. You should now see the authorization screen.