Add a form to filter notifications by recipient

Because manually editing the URL isn’t a great user interface, this
commit adds a search field to do this on the user’s behalf.

For this pass at the story it doesn’t do any validation – the user will
just get no results if they search by something which isn’t a phone
number or email address.

If the user navigates to a different ‘bucket’ of notifications (eg
delivered, failed) then the search term is reset, because they’ve
changed the filter which is at a level above the search term.

app/assets/stylesheets/components/page-footer.scss

@@ -59,3 +59,21 @@
   }
 
 }
+
+.align-button-with-textbox {
+
+  .button {
+
+    @include media(desktop) {
+      position: relative;
+      top: 32px;
+      left: -30px;
+      width: 100%;
+      margin-right: -30px;
+      padding-top: 8px;
+      box-sizing: content-box;
+    }
+
+  }
+
+}