ignore stale `users/show` responses - fixes #741

* person starts typing "abcd"
* request for user "abcd" is sent to server
* person continues typing "ef"
* request for user "abcdef" is sent to server
* response for user "abcdef" arrives, icon for user "abcdef" is shown
* response for user "abcd" arrives, icon for user "abcd" was shown –
  with this commit, this response is ignored instead

This is not just "showing the wrong avatar", btw: `MkSignin` uses the
`user` variable to decide whether to use 2FA, so having the wrong data
in user can make a login attempt fail.

packages/frontend/src/components/MkSignin.vue

@@ -110,7 +110,9 @@ function onUsernameChange(): void {
 	misskeyApi('users/show', {
 		username: username.value,
 	}).then(userResponse => {
-		user.value = userResponse;
+		if (userResponse.username === username.value) {
+			user.value = userResponse;
+		}
 	}, () => {
 		user.value = null;
 	});