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/assets/stylesheets/_grids.scss

@@ -26,6 +26,19 @@
   @include grid-column(7/8);
 }
 
+%top-gutter,
+.top-gutter {
+  @extend %contain-floats;
+  display: block;
+  margin-top: $gutter;
+  clear: both;
+}
+
+.top-gutter-4-3 {
+  @extend %top-gutter;
+  margin-top: $gutter * 4 / 3;
+}
+
 %bottom-gutter,
 .bottom-gutter {
   @extend %contain-floats;