Fixed display of warnings after content import (#2192)

- Fixes the horizontal overflow caused my longer warning/error messages. This means the <pre> tag is sized to fit the container and can now be scrolled and read in its entirety.
- Adds some top margin so these errors are not so bunched up to the text & input.

ghost/admin/app/styles/layouts/main.css

@@ -1327,6 +1327,7 @@
 .gh-expandable {
     background: var(--main-color-content-greybg);
     border-radius: 3px;
+    overflow-x: hidden;
 }
 
 .gh-expandable-block {

ghost/admin/app/styles/layouts/settings.css

@@ -1183,6 +1183,7 @@ body:not([data-user-is-dragging]) .gh-newsletter-card-draggable:hover .grab-news
     letter-spacing: 0.2px;
     background: #fff;
     border-radius: 5px;
+    margin-top: 18px;
     margin-bottom: 25px;
 }