Gave members table a sticky header

no issue

- it's useful to be able to see the column headers anywhere in the list
- prep for having the table header toggle between normal and edit modes where having to scroll back up to the top of the page each time you want to toggle would be a poor experience

app/styles/layouts/members.css

@@ -49,6 +49,16 @@
     table-layout: fixed;
 }
 
+.members-list .gh-list-row.header {
+    z-index: 1;
+}
+
+.members-list .gh-list-header {
+    position: sticky;
+    top: 84px;
+    z-index: 1;
+}
+
 .members-header .view-actions input.gh-members-list-searchfield {
     min-width: 220px;
     padding-left: 30px;
@@ -444,4 +454,4 @@ p.gh-members-import-errordetailtext {
 .gh-members-import-errordetailtext:not(:last-of-type) {
     padding-bottom: 4px;
     margin-bottom: 6px;
-}
+}