Add DoD Disclaimer

styles/atat.scss

@@ -42,6 +42,7 @@
 @import 'components/audit_log';
 @import 'components/usa_banner';
 @import 'components/checklist';
+@import 'components/full_page_notice.scss';
 
 @import 'sections/login';
 @import 'sections/home';

styles/components/_full_page_notice.scss

@@ -0,0 +1,16 @@
+.full-page-notice {
+  background: rgba(0,0,0,.5);
+  bottom: 0;
+  left: 0;
+  overflow: scroll;
+  padding: $gap*4 0;
+  position: fixed;
+  right: 0;
+  top: 0;
+  z-index: 999;
+
+  .usa-alert {
+    margin: 0 auto;
+    max-width: $article-max-width + $nav-width;
+  }
+}