Replace default Welcome Guide Popup with a Custom one

[yonkov]

By selecting a "featured" tour from the plugin's settings page, users should override the default Welcome Guide modal that pops up in the Gutenberg Editor.

Acceptance criteria: The featured tour should pop up when you open the editor for the first time

Useful links:

1. WordPress/gutenberg#19512
2. https://wordpress.org/plugins/disable-welcome-messages-and-tips/
3. Building custom Welcome modal in Wordpress: https://css-tricks.com/adding-a-custom-welcome-guide-to-the-wordpress-block-editor/