Tweak spacing on API keys page

Splits the clients section out a bit more clearly.

app/templates/views/api-keys.html

@@ -8,7 +8,7 @@
 
 {% block maincolumn_content %}
 
-  <div class="grid-row">
+  <div class="grid-row bottom-gutter-1-2">
     <div class="column-two-thirds">
       <h1 class="heading-large">
         API integration
@@ -54,11 +54,11 @@
     {% endif %}
   {% endcall %}
 
-  <div class="bottom-gutter">
+  <div class="bottom-gutter-2">
     {{ api_key(current_service.id, "Service ID", thing="service ID") }}
   </div>
 
-  <h2 class="heading-small">API clients</h2>
+  <h2 class="heading-medium">API clients</h2>
   <ul class="list list-bullet">
     {% for name, url in [
       ('Java', 'https://github.com/alphagov/notifications-java-client'),