Filtered notifications may stop returning new notifications if the filter hides too many

[sirbrillig]

The various filter behaviors supported by the app (Mute repo, Filtering by assigned, etc.) hide certain notifications from the UI. However, under the hood those notifications are still fetched and persisted locally. Since we only fetch the first 50 notifications (read and unread) from the Github endpoint, it's possible for there to be more notifications than we display. This is true all the time, but when filtering is applied, it's much more likely.

Let's say a user has 60 unread notifications and the first 40 of them are hidden because of filtering; you'd expect to see 10 show up in the UI. The app will only fetch the first 50 of those notifications (unfiltered), and because they are all filtered out, the UI will show no notifications at all.

[sirbrillig]

This is related to #96