feat: Implement simple Project Summary page (#71)

This implements a very simple Project Summary page (which lists workspaces):

![image](https://user-images.githubusercontent.com/88213859/151085991-bf5b101a-eadd-445b-9b42-1e98591e8343.png)

...which also has an empty state:

![image](https://user-images.githubusercontent.com/88213859/151086084-90d526a9-7661-46f0-b205-976518f978c1.png)

Fixes #66

site/util/array.test.ts

@@ -0,0 +1,17 @@
+import { firstOrItem } from "./array"
+
+describe("array", () => {
+  describe("firstOrItem", () => {
+    it("returns null if empty array", () => {
+      expect(firstOrItem([])).toBeNull()
+    })
+
+    it("returns first item if array with more one item", () => {
+      expect(firstOrItem(["a", "b"])).toEqual("a")
+    })
+
+    it("returns item if single item", () => {
+      expect(firstOrItem("c")).toEqual("c")
+    })
+  })
+})