#2956665 Site Activity: adds contextual popover menu item to add site credentials

Changes proposed in this Pull Request

• Adds contextual popover menu item to add site credentials in backup events on the site activity.
• Adds new calypso_activitylog_event_add_credentials tracks event for this new item.

We’re currently disabling the “Restore to this point” option on the site activity without explaining why, and I’d assume people would not co-relate the banner at the top telling them to add server credentials and the fact that restores are not available.

Before

After, normal state

After, selected state

After adding server credentials, the restore option is enabled and this one goes away

Testing instructions

• Fire up this PR locally or using jurassic.ninja.
• Connect Jetpack.
• Go to your site activity in Calypso; e.g.: http://calypso.localhost:3000/activity-log/SITE_URL.
• Ensure you see the new option and that it sends you to the correct place to set up site credentials.
• Validate that the option isn’t visible once you’ve added working credentials.