Notifications: page slow to load and results in time-out when many undeleted notifications

[sbarbosadataverse]

internal server error when trying to load notification page for sonia in production, likely due to trying to load a large number of notifications/need "Delete all" function
This is unusable for me at this time.

related to #2891

[bencomp]

This is probably due to the way notifications are rendered:

1. get all notifications for the current user
2. process every notification, i.e. do lookups for dataverse, dataset, datafile or user that the notification is about and print it

A user notification should be processed once, at the time that it is generated. If possible, notifications should be generated and processed asynchronously.

[raprasad]

I have the same issue on production under my own account.....