Improves cms page options in the adminhtml configuration

This issue was originally raised in https://github.com/magento/magento2/pull/10426 by @hostep but unfortunately he cannot continue at the current time. The aim is to improve the admin usability for multiple store setups and the selection of cms pages in the store configuration.

Preconditions

1. PHP 7.*
2. Magento 2.*

Steps to reproduce

1. Login to the Magento Admin with a multiple store setup,
2. Create multiple CMS pages with the same name, e.g. a Home Page for each of your stores,
3. Visit the configuration settings for "Default Pages",
4. See multiple "Home Pages"

Expected result

1. On the default store you will see only CMS pages attached to all store views and on a specific store you will see only CMS pages attached to that store.

2. Result should us UI component see https://github.com/okorshenko/magento2/pull/4/files for an example build.

Actual result

1. Hard to read simple drop down.

Original Report: https://github.com/magento/magento2/issues/11416 by @dmanners

Start at the Magento Admin configuration for "Default Pages" and review the original pull request 10426 and the linked example UI component. Confirm how CMS pages are selected for the default store versus a specific store view, then verify that duplicate page names are readable and limited to the applicable store scope.