Implement a basic notification system for users

[meghanmae]

UI part relies on #4 but can be started before #4 is done.

• Implement a notification system so that every time a new thread is added to a group the user has joined the user is notified.

• Implement a notification system so that every time a new comment on a thread the user posted is added the user is notified.

• Add UI to display these notifications

• Consider strong architecture patterns when writing this and note that more notifications (notifications when someone likes a comment, posts an event, joins a group you own, etc) will be added on in the future.

• Hold a quick standup to flush out the architecture around this

[meghanmae]

Children

• Allow muting notifications #13