#5428521 Add Custom Description for Rollback Fixer

Changes proposed in this Pull Request

• Jetpack Scan provides customized descriptions of how it will fix a threat. This PR adds a custom description for fixing threats with a “rollback”.
  Example: Jetpack Scan will rollback the affected file to the version from January 8, 2021, 3:40 am.

Testing instructions

• Use a test site with backup and scan plans
• Ensure the site has at least one successful, full backup
• Using the Threat Tester Plugin on your test site – add a “infect a file” threat. (You can get the plugin here)
• Run calypso locally and initiate a scan of the test site
• Verify that the threat was discovered and that a Fix is available
• The description of the fix should read something like Jetpack Scan will rollback the affected file to the version from <date-and-time>.