Configurable delay for nagging Heartbeat dialogs

[rehandalal]

Copied from: mozilla/normandy#431

mozilla/normandy#429 adds options for how the Heartbeat dialog re-prompts users. The nag option re-displays the notification until the user interacts with it in a meaningful way. However, this doesn't include closing the notification with the close button, so they will be re-prompted on startup again if they close the notification.

@MattGrimes has asked that we add the same kind of configurable delay as we have for the xdays option. This delay will control how long after a dismissal we should wait before re-prompting. The main use case here is things like prompting Beta users to move to release: we want to prompt them with the option to either decline or go to the Beta download page. If they choose neither and close the prompt, we wait some amount of days, and then re-prompt them.

See original issue for discussion.