Allow for narrow / square logo images for narrow screens

[choldgraf]

It is common for organizations to have both a "wide" and a "square" version of their logo for use in different contexts. We currently only support one style of logos, which means that usually the logo is either good on narrow screens but too small for wide screens, or vice versa.

For example, here's how our Team Compass scales with size:

We'd gain a lot of horizontal real estate if we supported a logo_narrow_dark and logo_narrow.

[agoose77]

This is a good suggestion @choldgraf. Right now, the theme needs a styling pass for the menu bar (especially since search was added). I did a pass to improve things, but I think it needs another deep investigation as to best practice for responsive layout.