Make Facebook login comply with best practices

[brianhelba]

Use a clearly branded "Log in with Facebook" button: https://developers.facebook.com/docs/facebook-login/checklist/#brandedlogin

When displaying the Log in with Facebook button, emphasize that your app will not post to Facebook without people's permission: https://developers.facebook.com/docs/facebook-login/checklist/#post

Ensure that pagination through long lists of Facebook groups is supported: https://developers.facebook.com/docs/graph-api/using-graph-api/#paging

Make a batched request for user profile + groups: https://developers.facebook.com/docs/graph-api/making-multiple-requests/#simple

Verifying Graph API Calls with appsecret_proof: https://developers.facebook.com/docs/graph-api/securing-requests/#appsecret_proof