Make the flow of using templates better

For users who:
- want to send messages from a template
- want to edit templates

For developers:
- who need to get the ID of a template

This commit mainly cleans up the choose template page so there are less
options, and the options that are there are less wordy.

This means:
- moving ‘send yourself a test’ onto the send messages page, and making
  it button
- stripping a lot of stuff out of the ‘send from API’ page, so it’s more
  obvious what the template ID is

app/templates/components/api-key.html

@@ -1,8 +1,8 @@
-{% macro api_key(key, name) %}
+{% macro api_key(key, name, thing="API key") %}
   <h2 class="api-key-name">
     {{ name }}
   </h2>
-  <div data-module="api-key" data-key="{{ key }}">
+  <div data-module="api-key" data-key="{{ key }}" data-thing="{{ thing }}">
     <span class="api-key-key">{{ key }}</span>
   </div>
 {% endmacro %}