admin: show the user last_activity the table is sorted by

The 'Last Activity' column rendered the spawner's last_activity while the
sort handler asks the API for ?sort=last_activity, which orders users by
User.last_activity. The two fields differ, so clicking the sort arrows
reordered rows in a way the column did not explain.

Render User.last_activity in the column; the per-server value is still
shown in the collapsed Server details.

jsx/src/components/ServerDashboard/ServerDashboard.jsx

@@ -428,7 +428,9 @@ const ServerDashboard = (props) => {
             <p className="text-secondary">{server.name}</p>
           </td>
           <td data-testid="user-row-last-activity">
-            {server.last_activity ? timeSince(server.last_activity) : "Never"}
+            {/* sorted server-side on the user's last_activity; the per-server
+                value is in the collapsed Server details below */}
+            {user.last_activity ? timeSince(user.last_activity) : "Never"}
           </td>
           <td data-testid="user-row-server-start-time">
             {server.started ? timeSince(server.started) : "Never"}

jsx/src/components/ServerDashboard/ServerDashboard.test.js

@@ -16,6 +16,7 @@ import regeneratorRuntime from "regenerator-runtime";
 
 import ServerDashboard from "./ServerDashboard";
 import { initialState, reducers } from "../../Store";
+import { timeSince } from "../../util/timeSince";
 
 jest.mock("react-redux", () => ({
   ...jest.requireActual("react-redux"),
@@ -818,6 +819,65 @@ test("Shows 'Never' for server start time when server never started", async () =
   expect(startTimeCell.textContent).toBe("Never");
 });
 
+test("Shows the user last activity that the table is sorted by", async () => {
+  // the column is sorted server-side on User.last_activity, so it has to show
+  // that field and not the spawner's
+  const userActivity = new Date(Date.now() - 5 * 60 * 1000).toISOString();
+  const serverActivity = new Date(
+    Date.now() - 3 * 60 * 60 * 1000,
+  ).toISOString();
+  const user = {
+    ...newUser("testuser"),
+    server: null,
+    last_activity: userActivity,
+    servers: {
+      "": {
+        name: "",
+        last_activity: serverActivity,
+        started: serverActivity,
+        ready: true,
+        url: "/user/testuser/",
+      },
+    },
+  };
+
+  useSelector.mockImplementation((callback) => {
+    return callback({
+      user_data: [user],
+      user_page: { offset: 0, limit: 2, total: 1 },
+      limit: 2,
+    });
+  });
+
+  await act(async () => {
+    render(serverDashboardJsx());
+  });
+
+  expect(screen.getByTestId("user-row-last-activity").textContent).toBe(
+    timeSince(userActivity),
+  );
+});
+
+test("Shows 'Never' for last activity when the user was never active", async () => {
+  const user = { ...newUser("testuser"), server: null, last_activity: null };
+
+  useSelector.mockImplementation((callback) => {
+    return callback({
+      user_data: [user],
+      user_page: { offset: 0, limit: 2, total: 1 },
+      limit: 2,
+    });
+  });
+
+  await act(async () => {
+    render(serverDashboardJsx());
+  });
+
+  expect(screen.getByTestId("user-row-last-activity").textContent).toBe(
+    "Never",
+  );
+});
+
 test("Displays profile information correctly", async () => {
   await act(async () => {
     render(serverDashboardJsx());