Clean up search box outline

This removes the default outline of the search box, which had curved edges
(going against the Playbook's implicit design principles) and was partially
obscured by the search button

Instead, since the input and button are tightly coupled (visually and
conceptually), the entire search form is outlined when focusing within the
search box, with a cleaner design using one of our brand colours. This is the
same colour as used for underlining links, so the colour consistently signals
a location of (potential) action (clicking a link; searching the playbook)

src/_sass/_search.scss

@@ -3,6 +3,10 @@
 .search-form {
   display: flex;
 
+  &:focus-within {
+    outline: solid 0.1875rem colours.$blue-light;
+  }
+
   &__input {
     background-color: colours.$ivory;
     border: 0.063rem solid colours.$navy;
@@ -11,6 +15,10 @@
     margin-right: -0.063rem;
     padding: 0.5rem 0.938rem;
     width: 100%;
+
+    &:focus {
+      outline: none;
+    }
   }
 
   &__submit {