Auto dismiss "Indices could not be archived yet" system notification

## What?

When an archiving error occurs Graylog creates a system notification:

The system continues to retry archiving, but if it eventually succeeds the system notification persists until a user manually dismisses it.

## Why?

From customer support request: https://github.com/Graylog2/support/issues/291
[gz#2354](https://graylog7416.zendesk.com/agent/tickets/2354)

Leaving the notification in its current state adds unnecessary confusion.