(Meetup use case) Add interface-based solution for saving / re-using the result of the session

[IoanaAMuresan]

In light of using Playground to effectively demonstrate the value of WP to first time users, it would be helpful to have a straight-forward way to keep the work done in the session.

This could be done by offering:

• a way to save a .zip backup to be used on any hosting
• a no-fuss upload to WordPress.com - similar to the one in Studio

Ideally - for ease of use in the meetup - these would be present directly in the interface, possibly on the Home tab, or - alternatively - under Tools.

If it's not helpful to have it in the general tool, these could be part of the 'filtered' meetup blueprint (https://github.com/WordPress/wordpress-playground/issues/1536)