Sharing dialog allows creating duplicate sharings

## Expected Behavior

If a sharing is configured for a user that already has one, it should be deduplicated/replace the previous one.

## Current Behavior

## Possible Solution

## Steps to Reproduce (for bugs)

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## Context

## Your Environment

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

## Checklist
[] This issue fix need to be backported.
[] Does this issue have **security** implications?