Added user button and menu.

components/UserButton.module.css

@@ -0,0 +1,17 @@
+.container {
+  display: flex;
+  position: relative;
+  cursor: pointer;
+}
+
+.icon {
+  margin-right: 8px;
+}
+
+.username {
+  border-bottom: 1px solid var(--gray500);
+}
+
+.username:hover {
+  background: var(--gray50);
+}