Make input work on older layouts

The bank holidays page is using an older layout, which includes the
button CSS. This contains a style rule on the `input` element, rather
than a class.

https://github.com/alphagov/static/blob/79d29b6a4221874ead27e67d007704a6
be166a57/app/assets/stylesheets/helpers/_buttons.scss#L91-L122

To make the styles work on these layouts we need to make the component
more specific (unfortunately). We also reset the margin to override the
styles from buttons.css

https://trello.com/c/8TC3AEms

app/assets/stylesheets/govuk_publishing_components/components/_input.scss

@@ -1,10 +1,17 @@
-.gem-c-input {
+// scss-lint:disable QualifyingElement
+.gem-c-input,
+
+// Explicitly set the input type so that we have higher specificity than
+// https://github.com/alphagov/static/blob/79d29b6a4221874ead27e67d007704a6be166a57/app/assets/stylesheets/helpers/_buttons.scss#L91-L122
+// This can be removed once everything is using `core_layout`.
+input[type=text].gem-c-input {
   @include core-19;
 
   box-sizing: border-box;
   width: 100%;
   height: 2.10526em;
-  margin-top: 0;
+
+  margin: 0; // Override unwanted global cascaded styles
   margin-bottom: 20px;
 
   padding: $gem-spacing-scale-1;
@@ -16,6 +23,7 @@
   // Disable inner shadow and remove rounded corners
   appearance: none;
 }
+// scss-lint:enable QualifyingElement
 
 @media (min-width: 40.0625em) {
   .govuk-c-input {