Add a new banner for telling you about trial mode

We’ve found in research that developers have no idea they’re in trial
mode until they hit an error. And even then they don’t really know what
trial mode means.

So this commit:
- adds a message to the API integration page about trial mode
- puts it in a really yellow banner to draw attention to it
- adds the same banner to the settings page

app/assets/stylesheets/components/banner.scss

@@ -221,3 +221,30 @@
   }
 
 }
+
+.banner-warning {
+
+  @extend %banner;
+  @include bold-19;
+  background: $yellow;
+  color: $text-colour;
+  border: 5px solid $text-colour;
+  margin: $gutter 0 $gutter 0;
+  text-align: left;
+  padding: 20px;
+
+  .heading-medium {
+    @include bold-24;
+    margin: 0 0 $gutter-half 0;
+  }
+
+  .list {
+    margin-bottom: 10px;
+  }
+
+  a:link,
+  a:visited {
+    color: $text-colour;
+  }
+
+}