vertically align non-square sponsor logos

[bridgetkromhout]

Expected behavior

All sponsor logos will be vertically aligned in a balanced-looking way.

Actual behavior

Sponsor logos are top-aligned and it looks uneven.

Reproduction Steps

The best example is https://www.devopsdays.org/events/2017-denver/welcome/ right now. See also:

[mattstratton]

Just making a note - the way to fix this is we should create a new class for .sponsor_image and set the formatting there.

[mattstratton]

I did a little hacking on this one, and it's beyond my CSS ability right now. It needs to be done, but it's not going to be trivial. Well, at least not trivial for me :)