Extensions page: add note for Update Now functionality when license is expired

[dashaluna]

The Update Now button appears on the Extension page when a new version of the plugin is available. However, it doesn't update the plugin if the license is expired / invalid.

Ref: see original discussion in: #1122

To Do:

• Make Update Now button disabled when license is invalid.
• Add clear "note" link to offer to purchase the license, ex Valid license is required to update.
• Add the "note" link to Tools -> Backups -> Extension page.
• Add the "note" link to Plugin general page.