auth: Introduce User struct

The `UserClaims` structure is an implementation detail of how the JWT
encoding process works.  We do not need to expose all of the details of
the JWT, such as issuer, audience, expiration, etc. to rest of the
application.  Route handlers should only be concerned with the
information about the user, rather than the metadata about how the user
was authenticated.

tests/integration/auth.rs

@@ -78,7 +78,9 @@ async fn test_login() {
     assert_eq!(location, "/");
     let cookie = res.cookies().get("auth.token").unwrap();
     debug!("Cookie: {:?}", cookie);
-    let claims = ctx.decode_jwt(cookie.value()).unwrap();
-    debug!("Claims: {:?}", claims);
-    assert!(!claims.sub.is_empty());
+
+    // Check to ensure the cookie contains a valid token
+    let user = ctx.decode_jwt(cookie.value()).unwrap();
+    debug!("User: {:?}", user);
+    assert!(!user.id().is_empty());
 }