Clarify login UI

[aaronpk]

I was a little disoriented by the page that asked me to choose between twitter and github OAuth
the green bar is clear...
but the buttons with checkmarks look like they're done. They don't invite clicking.
it wasn't until I read the text that I understood what to do
"Authenticate using one of the methods below"
and there's somehow not enough separation between the buttons I should choose between and the "You will be redirected to ..." stuff

https://chat.indieweb.org/2016-09-16#t1474044922192000