Improve help messages in SDG selector

Hopefully now it's a bit more obvious that SDGs can be selected by
clicking on them and that the field to select goals and the field to
select targets/goals are related (since they're now part of the same
fieldset).

app/assets/stylesheets/sdg/related_list_selector.scss

@@ -2,6 +2,10 @@
   @include grid-column-gutter;
   clear: both;
 
+  > legend {
+    margin-bottom: 0;
+  }
+
   .amsify-suggestags-area .amsify-select-tag {
     color: $white;
 
@@ -17,7 +21,8 @@
     @include sdg-goal-list($spacing);
 
     legend {
-      @include element-invisible;
+      font-weight: normal;
+      font-style: italic;
     }
 
     label {
@@ -46,6 +51,11 @@
     }
   }
 
+  label {
+    font-weight: normal;
+    font-style: italic;
+  }
+
   .input-section {
     margin-bottom: 2 * $line-height / 3;
   }