Change signin control from text list to buttons.

[johncox-google]

@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.

[marcotuts]

Thanks @johncox-google for the work done here. I'm spinning this up locally for testing and any potential suggestions, but I did a pure FED code run through and had a couple suggestions. Will note them inline on the last commit. https://github.com/johncox-google/edx-platform/commit/4dd0999eac115cdd1a9276d89a0f4f8acbe0f59d

[marcotuts]

👍 on button styling

[johncox-google]

@nedbat Since this PR is a superset of https://github.com/edx/edx-platform/pull/2994 and https://github.com/edx/edx-platform/pull/3408 I did a soft reset off master so we can do one merge rather than three.

While doing final checks I also found a couple corner cases and fixed them in 1e052e3. Could you have a look? If you're happy with the change I'll squash/rebase so you'll be clear to merge.

Thanks, and sorry for the last minute churn.

[johncox-google]

@nedbat Cleared up dispatch logic in 1192b5f.

[johncox-google]

Forgot to tag @psimakov to keep him in the loop.