Replace test with one-off

It’s a confusing proposition to have two features which are almost
identical. Even differentiating between them in the template menu would
be tricky.

I think the better thing to do is rename the whole feature to ‘send
one-off message’.

Then if someone wants to use there own phone number or email address,
give them a quick shortcut to doing that, once they’re in the flow.
In the background this reuses the ‘send yourself a test’ code, but
the user is never aware that they’re going through a different route
to send an individual message. So the proposition stays nice and clean.

app/templates/views/send-test.html

@@ -15,10 +15,20 @@
   </h1>
 
   <form method="post" class="js-stick-at-top-when-scrolling" data-module="autofocus">
-    {{ textbox(
-      form.placeholder_value,
-      hint='Optional' if optional_placeholder else None
-    ) }}
+    <div class="grid-row">
+      <div class="column-two-thirds">
+        {{ textbox(
+          form.placeholder_value,
+          hint='Optional' if optional_placeholder else None,
+          width='1-1',
+        ) }}
+      </div>
+      {% if skip_link %}
+        <div class="column-one-third">
+          <a href="{{ skip_link[1] }}" class="top-gutter-4-3">{{ skip_link[0] }}</a>
+        </div>
+      {% endif %}
+    </div>
     {{ page_footer('Next', back_link=back_link) }}
   </form>