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/assets/stylesheets/components/api-key.scss

@@ -11,10 +11,7 @@
     margin-bottom: 10px;
   }
 
-  &-button-show {
-    @include button($grey-3);
-  }
-
+  &-button-show,
   &-button-copy {
     @include button($grey-3);
   }