feat(users): Update social login design

[benobab]

New design for social links

• responsive images
• grid for mobile layout as well

[mleanos]

LGTM. I've tested this as well.

Thanks @benobab!

[hyperreality]

One small gripe I have with this commit is that it works great if you want to authenticate with all 6 networks, but in most cases that won't be true, and after removing a button or two it's hard to get a nice centred design.

Furthermore, I tried shrinking the page in Firefox and below a certain size it's not possible to click the buttons at all.

Also why we're at it why don't we make the buttons interactive? Add them to a 'social' class and put this in core.css:

.social {
   -webkit-transition-duration: 0.4s;
   -moz-transition-duration: 0.4s;
   -o-transition-duration: 0.4s;
   transition-duration: 0.4s;
}

.social:hover {
    opacity: 0.65;
}

[benobab]

@hyperreality I've updated the code with your css and fixed the bug where you can't click the buttons at all (xs rather than md for bootstrap).

[lirantal]

Thanks a lot @benobab, I fixed the conflict and added to your commits a new PR which can be merged at: #1450

[lirantal]

closed through PR #1450