Add support for user notifications

[micahflee]

There should be some sort of way to communicate to the user when there's a persistent problem. For example, if the Flock server consistently returns errors.

[micahflee]

Additionally, some actions should result in the security team getting an immediate notification, for example detecting a reverse shell. The user should also get an immediate notification. The UX for this needs to be considered carefully, especially because some notifications could false positives.

[micahflee]

I think there should be a notifications tab that has a list of notifications, the most recent at top. Each notification can have:

• Type, and link to wiki for more info about this notification type
• Severity (green, yellow, or red)
• Details
• Timestamp

When a notification happens, it should maybe update the systray icon to include a red circle in the corner, meaning there are unread notifications, and popup a system notification message in the top-right corner with the notification, and when you click it it shows the window and selects the notifications tab.

Users should be able to clear notifications from the notifications tab.

[micahflee]

Every 2 minutes, the agent should make an authenticated API request to /heartbeat (firstlookmedia/flock-server#18) to retrieve a list of notifications to add to the notifications tab.

Additionally, /heartbeat will respond with the user's name. If the user's name has changed, then it should update it to the new name (#39).

[micahflee]

Alternatively, user notifications can be in the form of messages via an encrypted messaging app like keybase or signal...