UI. Add Backup Management toggle to PMM settings

User story:

• As a PMM Admin, I need to be able to turn ON/OFF backup functionality in the PMM Server so I can control its usage by PMM users

UI/UX:
"Usual" Feature toggle

Acceptance criteria

• Backup is OFF by default
• Backup functionality is controlled via UI+

Suggested implementation:
TBD

How to test:

• To de(activate) Backup Management, go to PMM Settings, Advanced tab;
• Without Backup Management enabled, user should not see the Backup icon (clock-like icon) on the sidebar. Also, by going to `/graph/backup`, user should see the Backup page with a warning that allows the user to go to PMM Settings (see screenshot);
• By activating Backup Management, the page reloads and the icon on the sidebar becomes visible. Also, user can go to the page and use it normally.

Details:
Backup in PMM in a Technical Preview, so people need to turn the feature explicitly and able to OFF it if needed