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() %}

app/templates/views/send.html

@@ -28,7 +28,7 @@
     ) }}
   {% endif %}
 
-  <p>
+  <h2 class="heading-medium">
     You need
     {{ template.placeholders|length + 1 }}
     {% if template.placeholders %}
@@ -37,7 +37,7 @@
       column
     {% endif %}
     in your file, like this:
-  </p>
+  </h2>
 
   {% call(item, row_number) list_table(
     example,