Change signin control from text list to buttons.

@marcotuts: I had a bit of spare time so I took a stab at changing the signin controls for social auth from list of text to list of buttons. I know this is something you talked about doing; if you've already done it, let me know and I'll back this out. Otherwise, I'd like to merge it in this week along with https://github.com/edx/edx-platform/pull/2994 and https://github.com/edx/edx-platform/pull/3408.

There's a bit of repeated code because I couldn't figure out how to make vars in a parent mako template visible to a child mako template.

Anyway, this is based off https://github.com/edx/edx-platform/pull/3408 and should not be merged before it. All relevant changes are in 4dd0999.

@nedbat, @jzoldak: if @marcotuts likes the UX, this would resolve one of @jzoldak's concerns at https://github.com/edx/edx-platform/pull/2994#issuecomment-41180064.