Enhance dashboard styles (#16884)

* Display sparkline graph on Chrome

* Heatmap auto overflow

* Change grid columns number on small screen

* Please codeclimate bot

* Remove graph height

app/javascript/styles/mastodon/admin.scss

@@ -920,6 +920,13 @@ a.name-tag,
 }
 
 .retention {
+  overflow: auto;
+
+  > h4 {
+    position: sticky;
+    left: 0;
+  }
+
   &__table {
     &__number {
       color: $secondary-text-color;
@@ -1018,6 +1025,7 @@ a.name-tag,
   &__graph {
     position: absolute;
     bottom: 0;
+    width: 100%;
 
     svg {
       display: block;