Set font-family on buttons to NTA temporarily

Until we change all the buttons to use GOVUK
Frontend we need this CSS to give them the New
Transport (NTA) font-family.

Was previously in GOVUK Template CSS but missed
out from being moved across when this was removed:

https://github.com/alphagov/govuk_template/blob/f164b2825b1479d7c7d4bdbec4ba9c8e171f504c/source/assets/stylesheets/_basic.scss#L6

app/assets/stylesheets/globals.scss

@@ -1,3 +1,5 @@
+@import 'settings/typography-font-families';
+
 // needed for IE10 desktop snap mode: http://menacingcloud.com/?c=cssViewportOrMetaTag
 @-ms-viewport {
   width: device-width;
@@ -88,3 +90,10 @@ button:focus {
   outline: 3px solid $focus-colour;
   outline-offset: 0;
 }
+
+// To be removed when all buttons follow the GOV.UK Frontend conventions
+// - https://design-system.service.gov.uk/components/button/
+
+button {
+  font-family: $govuk-font-family-nta;
+}