Make heading sizes more consistent

On:
- the API keys page
- the ‘send’ page

There are some inconsistently small heading sizes. This commit makes them
consistent with the heading sizes on other pages.

app/templates/views/api-keys.html

@@ -22,7 +22,7 @@
     developer documentation</a>.
   </p>
 
-  <h2 class="api-key-name">
+  <h2 class="heading-medium">
     Service ID
   </h2>
   <p class="api-key-key">
@@ -33,7 +33,6 @@
     keys,
     empty_message="You haven’t created any API keys yet",
     caption="API keys",
-    caption_visible=False,
     field_headings=['Key name', hidden_field_heading('Action')]
   ) %}
     {% call field() %}