Inform Users of First Backup/First Restore Point for Paid Plans

[drwpcom]

Issue:

After a site goes AT or a user upgrades Jetpack to a paid plan, it is unclear to users at what point backups become available.

Possible solution:

Show a message to users informing them that the first backup is running and changes made before its completion cannot be undone. Possible to options to do this include:

• a persistent message in WP Admin until the backup is done since it's likely that users will be making changes after installing their first plugin/custom theme.
• a one-time message on the post plugin/theme install page in Calypso saying the backup takes some time to complete.

This issue is a result of a conversation between @kriskarkoski, @tmmbecker, @seear and me in 3814-gh-jpop-issues.