feat: saml login

[gempain]

@nikhiljha we've merged meli-ui and meli into a single repo so it's easier to contribute. We'll most likely move the docs in a near future. I've taken code you've written in getmeli/meli-ui#3 and #158 and will be testing this before merging. I don't want to steal away the fame, so credits go to you 😉

closes #8, #158