Twenty Seventeen: Lots of space under blog post titles with one column layout on front page

[laurelfulford]

I noticed this while reviewing #40264:

When you set the blog posts page as one of the panels on the front page, and set the site to use the one column layout, there's a lot of space underneath the blog post titles:

Steps to reproduce:

1. On a site running Twenty Seventeen, set up a static front page
2. Pick your blog posts page as one of the panels
3. Set the theme to use the one column layout.

The spacing should actually be closer to what it's like in the two column layout:

Will follow up with a patch shortly.

[laurelfulford]

Makes blog header margin more specific on front page when layout is set to one column.