Fix "You have a lot of notifications" reminder with read notifications

[rollerozxa]

Fixes the "You have a lot of notifications" reminder to not count read notifications.

Not tested, but looks like how it should be from this other code for getting unread notifs:

contentdb/app/templates/base.html

Line 112 in 02aef8b

{% set notification_count = current_user.notifications.filter_by(read_at=None).count() %}