notification/slack: add interactivity to alert notifications

[mastercactapus]

Description:
Adds two-way interactivity as an option for Slack notifications.

Additional Info:
Summary of changes:

• make alert log store support both contact methods and notification channels for ack/close actions
• new http route /api/v2/slack/message-action added
• new config option Slack.InteractiveMessages to opt-in
• action support in the mockslack server
• new ReceiveSubject method that can map a provider/subject ID to a GoAlert user for notification channel responses
• remove alert as a dependency for the notification package by using a new AlertState type instead of alert.Status
• updated Send() method in Slack ChannelSender to add action blocks
• added support for Ephemeral messages and actions and assertions in test harness/mockslack
• added slack interaction smoketest
• added FindOneBySubject to user.Store
• added FieldErrorf to validation package to support formatting error messages cleaner

[arurao]

LGTM 🎉