Fix notifications on the dashboard

Notifications should be ordered new → old (https://edugit.org/AlekSIS/official/AlekSIS-Core/-/blob/master/aleksis/core/views.py#L188)
a notification doesn't have an app attribute, it is called sender (https://edugit.org/AlekSIS/official/AlekSIS-Core/-/blob/master/aleksis/core/templates/core/index.html#L88)

Edited Nov 16, 2021 by Jonathan Weth