Specify styles are meant for the main header

The <header> tag is a standard HTML tag which can be used in any section
of the page. We were adding the main header styles to all <header> tags
because in the past it was in only <header> tag we used.

That's not the case anymore, so instead of overwriting these styles on
every other <header> tag, modifying the rule so it only selects the main
header makes the code easier to maintain.

app/assets/stylesheets/layout.scss

@@ -501,7 +501,8 @@ a {
 // 02. Header
 // ----------
 
-header {
+body > header,
+.wrapper > header {
   background: $brand;
   border-bottom: 1px solid $border;
   margin-bottom: $line-height;

app/assets/stylesheets/legislation_process.scss

@@ -996,12 +996,6 @@
     @include grid-row;
     margin-top: $line-height * 1.5;
     padding: 0 rem-calc(16);
-
-    > header {
-      background: none;
-      border: 0;
-      margin: 0;
-    }
   }
 
   h4,