Implement basic auto-save

Overview

#3475672: Research: Possible backend implementations of auto-save, drafts, and publishing in we are discussing exactly how auto-save should work.

For now make using the demo easier this issue will be implement a very basic auto-save functionality

For demo purposes this will be mean

1. You can come back to an article and start where you left off
2. You can easily create multiple articles with different XB data

Proposed resolution

Save the front-end Layout and Model to a shared tempstore.

This means that for each node that uses XB the state of experience builder will be save when you reload browser. It will be shared among all users of XB for the node.

There will not be a way to clear the auto-save but this MR also updates toolbar XB item to link the current node if it uses the XB demo field. That way to start with an empty canvas you can just create another article

To delete all auto-save states you can drush sql:query "delete from key_value_expire where collection='tempstore.shared.experience_builder.auto_save"

After #3475672: Research: Possible backend implementations of auto-save, drafts, and publishing is finished we can do the actual auto-save functionality

User interface changes