Fixed breadcrumb not working at narrow widths and member activity scroll gap

refs: https://github.com/TryGhost/Team/issues/1277

- added a z-index to fix breadcrumbs when filters overlap
- made the scrolling list longer on the member activity page to remove gap

ghost/admin/app/styles/layouts/main.css

@@ -1394,6 +1394,7 @@
 
 /* Canvas */
 .gh-canvas-title {
+    z-index: 1;
     display: flex;
     align-items: center;
     overflow: hidden;

ghost/admin/app/styles/layouts/member-activity.css

@@ -3,6 +3,10 @@
     border-top: 1px solid var(--whitegrey);
 }
 
+.gh-members-activity .gh-list-scrolling {
+    height: calc(100vh - 96px);
+}
+
 .gh-members-activity .gh-list-scrolling thead th:last-child {
     width: 25%;
 }