jumbotron: tweak margin and padding

This matches the behavior before dropping to use the Bootstrap jumbotron
component.

static/assets/home.css

@@ -19,11 +19,17 @@
 
 #jumbotron {
   flex-shrink: 0;
-  padding: 4rem 0;
-  margin-bottom: 2rem;
+  padding: 2rem 0;
+  margin-bottom: 1rem;
   background-color: #e9ecef;
 }
 
+@media (min-width: 576px) {
+  #jumbotron {
+    padding: 4rem 0;
+  }
+}
+
 #jumbotron-content {
   position: relative;
   z-index: 1;