Content area extends beyond right gutter; add gutter classes where needed

To address margin issues, Fusion added gutter classes, used in places instead of the inner class. Relevant issues: #1151892: Add .gutter to #header-site-info-inner, #1002072: Width of the content div and blocks-system-main div.

Currently in Mix and Match the main content block is supposed to be full width but is inside divs that have margins. The result is that the content area (inside #block-system-main) spills over the right-hand gutter. A margin-right has been added to try to compensate, but this breaks in various contexts.

On a local site, I used the following to temporarily fix:

#content-inner-inner {
  margin: 5px 0;
}

#content-inner-inner > h1.title {
  margin-left: 10px;
  margin-right: 10px;
}

#content-content .inner {
  margin-left: 0;
  margin-right: 0;
}

#block-system-main > * {
  padding-left: 10px;
  padding-right: 10px;
}

But that only treats the symptom--what's needed is to update to the Fusion changes.