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

#### 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.

Start by locating the backup-status and post plugin/theme install entry points in WP Admin and Calypso. Review how first backups become available for paid plans, then define the user message and its visibility until the first backup completes, including the warning about earlier changes.

Written by the indexing model from the issue text.