Deleted searches/dashboards should not show up in "Last Opened" section of Welcome Page

## Expected Behavior

The "Last Opened" section is currently displaying saved searches/dashboards which have been opened recently but deleted afterwards as something like "Unknown entity: 6448d3f147326d7bfd5176d5". This is not really helpful for a user and should be omitted.

## Current Behavior

## Possible Solution

## Steps to Reproduce (for bugs)

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## Context

## Your Environment

* Graylog Version:
* Java Version:
* Elasticsearch Version:
* MongoDB Version:
* Operating System:
* Browser version: