fix(styles): stop wrapper from overflowing on smaller devices (#4121)

When using `swagger-ui` for consumption by JavaScript web projects
that include module bundlers, instead of `swagger-ui-dist`
there is no `box-sizing: border-box` on `.wrapper`
and it's width overflows because of additional `padding: 0 20px`

Author: Andrew Luca

src/style/_layout.scss

@@ -4,6 +4,7 @@
     max-width: 1460px;
     margin: 0 auto;
     padding: 0 20px;
+    box-sizing: border-box;
 }
 
 .opblock-tag-section

src/style/_mixins.scss

@@ -171,4 +171,4 @@ $browser-context: 16;
     animation: shake .4s 1;
     border-color: $_color-delete;
     background: lighten($_color-delete, 35%);
-}
+}