Change now assignment notifications are displayed

[thequbit]

Notifications for new assignments should only be displayed twice:

1. The first time the assignment is seen after hitting the JSON endpoint
2. If there are less than 24 hours until the assignment expires.

I think we should achieve this by using a running list of assignments seen, and only display the notification if one of the above to cases is met.

[thequbit]

Debjit, do you think you could work on this issue next?

Additionally, if someone responds to an assignment, it should not pop up again.

[thequbit]

Also, this same ticket needs to be opened on ios. I'll do that this afternoon/evening.