FIX: Navbar margin on mobile

[choldgraf]

This applies the same fix as #526 but in a more generic fashion. I realized that the problem was not unique to having a text-based site title. On mobile this is how it would look:

(note the mis-alignment with the content block below for the logo, and how the dropdown button is right up against the screen edge on the right)

With this fix, we add margin to the navbar-start and navbar-end div elements, so that no matter what is inside them, they'll have nicer padding. Here's how it looks on mobile now:

[damianavila]

This change looks good to me on mobile but I see a small linear and horizontal movement in the logo when I go below 960 px width (from a wider view). Probably not a blocker... just something I saw at the time to test this one.

[choldgraf]

I think that it's OK to have the padding slightly more on wide screens - but happy to discuss that in a follow up issue. Thanks @damianavila for the review!